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WWII Scrambler Phone
Frequency Changer · Privacy Set · Secraphone

The Frequency Changer, by Winston Churchill referred to as the Scrambler Phone, was a tele­phone privacy system, developed in 1939 by the British General Post Office (GPO, later: BT) at Dollis Hill (London, UK). Based on inversion of the voice spectrum, it consisted of two parts: a modified regular telephone set – commonly with a green handset 1 – that was used as the voice terminal, 2 and the actual scrambler, known as the Frequency Changer. It was made by various manufacturers, and is also known by the post-war names Secraphone and Privacy Set [1].

The scrambler was housed in a wooden or metal enclosure that was usually placed under the table or in an adjacent room. The voice terminal was placed on the desk of the calling parties. It came in several flavours, but was usually based on an adapted standard black 394 or 396 telephone, modified with a 164 handset in Jade Green. Due to wartime shortages, black handsets were sometimes painted lime green as an alternative.

The image on the right shows a typical SA5063 voice terminal with green 164 handset, that was used with the Scrambler from 1943 onwards [1].
  
SA 5063/1 voice terminal

During World War II, many scrambler systems were installed to prevent accidental or intentional eavesdropping. Initially, the public telephone network was used for this, but the War Office later established its own private network, that was completely independent from public exchanges. 3

The WWII image on the right shows British Prime Minister Winston Churchill behind his desk at the Cabinet War Rooms 4 just 10 feet below street level under the New Public Offices 5 at Whitehall. To his left is Royal Navy Captain Richard Pim who uses the Scrambler Phone on Churchill's desk.

In 1938, after a survey by the Office of Works, this building was thought to be suitable for use as a temporary office in the event of war. It was hastily converted into a reinforced 6 temporary command center and became operational in August 1939, just before the outbreak of WWII.
  
Churchill in the Cabinet War Rooms during WWII, with a staff member operating his Scrambler Phone. Copyright IWM [4]. Click to enlarge.

The Frequency Changer (scrambler) was developed in 1939 at the Post Office Research Station at Dollis Hill (North-West London). It was first deployed at home (in the UK) and in the field (abroad) in early 1940, with modified 4-wire variants being available for use over radio, and tropicalised versions for areas with high humidity. During the war, most units were manufactured at the GPO's Holloway factory in North-London (manufacturer code FH). After the war, production was moved to TMC in London – who marketed the system as Secraphone – and to other manufacturers [1].

  1. Green handsets were used to distinguise secured lines from regular ones, especially during the war. In the 1960s telephone sets with no distinct colour were used.
  2. In GPO terminology, the voice terminal is commonly known a the telephone instrument.
  3. Most of the War Office's private telephone networks were of the Local Battery (LB) type and were manually switched, which is why the vast majority of voice terminals does not have a dial.
  4. The Cabinet War Rooms — today known as the Churchill War Rooms — are now part of the Imperial War Museum (IWM) in London (UK) and are open to the public. It also houses the Churchill Museum [16].
  5. This building now houses the Treasury.
  6. Although this 'bunker' was reinforced several times during the war, it hardly offered any real protection against a direct bomb hit.

Voice terminals
Telephone No. 162 (1940)
162
Telephone No. SA5030 (1940)
Telephone No. SA5031 (1940)
Telephone No. SA5063/0 (1943)
Telephone No. SA5063/1 (1944)
Telephone No. 710 (1964)
710
Telephone No. 740 (1968)
740
 Introduction


Scramblers
Frequency Changer No. 6 (1940)
Frequency Changer No. 6A (1940)
Frequency Changer No. 6B (1940)
Frequency Changer No. 6AA/0 and 6AA/1 (1942)
Frequency Changer No. 6AC (1944)
Privacy Set No. 7 (1957)
Privacy Set No. 8 (1962)
Privacy Set No. 9 (1964)
Privacy Set No. 9
 Introduction



Accessories
Local relay unit for up to 3 voice terminals
Indicator No. 401CN (doll's eye)
Telephone connection box BT20/8
Telephone connection box BT No. 6
HELP REQUIRED — Crypto Museum are still looking for additional information and circuit diagrams of the above scrambler systems. We are also looking for a wartime valve-based frequency changer and for a post-war Telephone No. 710/740 that was used with Privacy Set No. 8 or 9. If you can provide any of these, please contact us.
Operation
The diagram below shows how the scrambler works. At the right is the 2 or 4-wire line to the exchange or to a radio. At the left is the voice terminal, which can be an individual microphone and speaker, a handset or a telephone. The audio signal from the microphone is first attenuated to reduce its dynamic range. It then passes a low-pass filter, so that only the 20-2000 Hz part of the spectrum is fed to a ring mixer, where it is added to the 2500 Hz signal from an oscillator.

At the output of the mixer, the sum and the difference of the two signals are available, with the difference being the mirrored version of the original signal (here shown in red). This means that low-frequency tones have become high-frequency tones and vice versa. After filtering it again in a low-pass filter, only the mirrored signal is left, which is then amplified and delivered to the line.

Block diagram of the Frequency Changer. Click to see the original drawing [1].

The bottom half of the diagram shows the reception path, which is more or less the same, but in reverse direction. The mixer produces two images again, of which the lower one is the mirrored version of the received signal. After filtering, the original audio signal remains, which is then amplified and delivered to the speaker. The spectrum diagrams should illustrate what happens.

Although the Frequency Changer, or frequency inverter, offered reasonable protection against an occasional (un)intentional eavesdropper, such as the exchange operator or a service engineer working on the lines, it was no match for a professional interceptor. All one had to do, was find the inversion frequency and mirror the spectrum again. A classical case of security by obscurity.

 More about scramblers


Setup
The diagrams below show the different configurations in which the scrambler could be used. In all cases 1 , the push-buttons on top of the telephone set (i.e. the voice terminal) are configured in such a way that in SECRET mode, the microphone and speaker are wired directly to the scrambler.

Using the Scrambler Phone over a 2-wire subscriber line

The diagram above shows the most common configuration, in which a regular (switched) 2-wire subscriber line is used to connect the two parties. In this situation, the fork circuit inside the scrambler unit is used to combine the transmit and receive circuits onto a single 2-wire line.

Using the Scrambler Phone over a 4-wire leased line

In addition, when 4-wire leased lines are available, it is possible to avoid the use of a fork circuit and use two separate 2-wire lines for transmit and receive. Although this solution provides the best possible audio quality, 4-wire switched lines were hardly used as they were very expensive.

Using the Scrambler Phone over a full-duplex radio link

It was also possible to use the scrambler system over wireless (radio) links. In that case, the units were used in 4-wire configuration, so that the voice circuits could be wired directly to the transmitter (TX) and receiver (RX). The diagram above shows a full-duplex scrambled radio link.

  1. With some versions of the scrambler it was possible to connect a standard (2-wire) telephone set instead of the (4-wire) handset by using an extra fork circuit, but in practice this was rarely done.




SA 5063/0 (left) and SA 5063/1 (right)

Voice terminals
Below is an overview of the various (converted) telephone sets that were used in combination with the Frequency Changers (scramblers). Note that many variations exist, often caused by shortages during the war. In the overview below, we are heavily drawing on the information supplied by Andy Grant in the UK, most of which was published in an article in Telecommunications Heritage Journal in the Summer of 2017 [1]. Many thanks to Andy for permission to use this information.

In the text below, Telephone Set Type Number is abbreviated to Tele.No.

Model Year Tele. Key Wires Block Remark
162 1940 162 - - - + black Tele.No.328 for initiating call
SA5030 1940 328 303A 6 BT20/8 CB/Auto
SA5031 1940 394 1 303A 8 BT20/8 LB (CBS 1, 2 & 3), Magneto
SA5063/0 1943 394 1 303A 12 BT No 6 LB (CBS 2 & 3), Magneto, Direct/PBX
SA5063/1 1944 394 1 303B 12 BT No 6 LB (CBS 2 & 3), Magneto, Direct/PBX
710 1962 710 2 - ? ? CB/Auto, 2 buttons
740 1968 740 2 - 10 BT37B CB/Auto, up to 4 buttons
  1. The chassis of a Tele.No. 396 was sometimes used as a replacement for the 394.
  2. Telephone No. 740 was used as an alternative to the 710.

Telephone No. 162   —  wanted
The first scrambler sets that were released in early 1940, used a green bakelite Tele.No.162 as the voice terminal for scrambled conversations. The call was initiated with a standard black telephone set of the No. 328 range, that was also connected to the Frequency Changer [1].

 More information

  
GPO 200 series telephone set in Jade Green, which is very similar to the Tele. No. 162. Photograph obtained from www.designc20.com

SA5030
Introduced in August 1940 for use on CB/Auto installations. It consists of a Tele. No. 328, fitted with a Key No. 303A switch assembly, a 6-wire line cord ending in a BT20/8 connection box, or Block Terminal, and a green Handset No. 164.

The set was usually fitted with two push-buttons – in front of the handset – labelled SECRET and NORMAL (later: SCRAMBLE and NORMAL).

 More information

  
SA5030 voice terminal

SA5031
Introduced August 1940 for use on LB (CBS 1, 2 & 3) and Magneto systems. It consists of a Tele. No. 394, fitted with a Key No. 303A switch asssembly, an 8-wire line cord, a BT20/8 connection box, and green handset 164. It is fitted with two push-buttons, labelled SECRET and NORMAL.

 More information

  
Example of an SA 5031 voice terminal. Photograph kindly supplied by Andy Grant [1].

SA5063   SA5063/0
Introduced in early 1943 for use on LB (CBS 2 & 3) and Magneto systems, and on for use on long lines with CB/Auto systems. It consists of a Tele. No. 394 with a Key No. 303A switch assembly, a 12-wire line cord ending in a BT No.6 connection box, and a No. 164 Jade Green handset [1].

 More information

  
SA 5063/0 voice terminal

SA5063/1
Introduced in February 1944 for use on LB (CBS 2 & 3) and Magneto systems, and on for use with long lines on CB/Auto systems. It consisted of a modified Tele.No.394, fitted with a Key No.303B switch assembly, a 12-wire cord ending in a BT No.6 connection box, and a green 164 handset.

 More information

  
SA 5063/1 voice terminal

Telephone No. 710
In 1964, two years after the introduction of the transistorised Privacy Set No. 8, the GPO gradually switched from the bakelite SA-5030 (based on the 300-series) to modified sets from the 700-series, which had a plastic body.

The first model was based on Telephone No. 710 (the successor to the 706), and had provisions for up to four push-buttons. Two of these buttons were used to switch between clear and scrambled speech.

 More information

  
Telephone No. 710. Copyright Gildings Auctioneers [10].

Telephone No. 740
In 1968, Telephone No. 740 was introduced as the successor to the 710. It has a slightly modified design and an improved electric circuit, but is otherwise nearly identical.

Note the unusual combination of a black body and a brown handset, which was probably done to indicate that this device was used for a special (i.e. non-standard) purpose.

 More information
  
GPO Telephone No.740





Overview of the various types of Frequency Changers and Privacy Sets

Frequency Changer mainframes   scramblers
There were several versions of the scrambler, depending on the application and on advances in technology. During WWII, all scrambler mainframes were known as Frequency Changer, whilst after the war, from 1957 onwards, the name Privacy Set was most commonly used. In addition, the names Privacy Unit and Secrecy Unit also turn up in literature every now and then. The British manufacturer TMC marketed the equipment under the name Secraphone [1].

Model Year Case System Valves Remark
Frequency Changer No. 6 1940 Wood CB/Auto CV1732 200-250V AC/DC 1
Frequency Changer 6A 1940 Wood CB/Auto/LB CV1732 200-250V AC
Frequency Changer 6B 1940 Wood CB/Auto/LB CV1732 200-250V AC/DC 1
Frequency Changer 6AA/0 1942 Metal CB/Auto/LB CV1052 3 200-250V AC
Frequency Changer 6AA/1 1942 Metal CB/Auto/LB CV1052 3 100-110/200-250V AC
Frequency Changer 6AC 1944 Metal CB/Auto/LB CV1052 3 100-250V AC + 12V DC 2
Privacy Set No. 7, 7A 1957 Metal CB/Auto/LB CV138 Improved circuits
Privacy Set No. 8, 8A, 9, 9A 1962 Metal CB/Auto 4 Transistorised
  1. Parts of the chassis carry a (potentially lethal) live voltage.
  2. Uses a vibrator pack.
  3. Equivalent to the EL32.
  4. Uses Germanium PNP transistors instead of valves.

Frequency Changer No. 6   —  wanted
Introduced in June 1940, for use on CB/Auto installations. The unit was housed in a wooden case and operated on 200-250V AC or DC mains power. It is built around CV1732 valves (tubes), which are 5-pin triodes designed for signal processing (equivalent to ML4 and VT90/VT129).

No. 6 was short-lived and was succeeded within months by the improved 6A and 6B versions, as it had several design issues.

 More information

  

Frequency Changer No. 6A   —  wanted
This improved version was introduced in August 1940 for use on CB/Auto or LB installations. It was suitable for the 200-250V AC mains only and had an isolated power transformer, which made it much safer for maintenance engineers. Like the No. 6 it is built around CV1732 valves.

 More information

  

Frequency Changer No. 6B   —  wanted
Introduced in September 1940, this version was nearly identical to the 6A, but was suitable for the 200-250V AC or DC mains and had therefore a live voltage-carrying chassis (like the No. 6).

This version was intended for use in areas that still had a Direct Current (DC) mains power network, with the ability to run from Alternating Current (AC), once the local mains network had been converted.

 More information

  

Frequency Changer No. 6AA   —  wanted
Introduced in 1942 for use on CB/Auto and LB installations. Unlike its predecessors, it was built around the CV1052 signal processing valve; an octal-based penthode equivalent to the EL32.

It came in two flavours: 6AA/0 and 6AA/1, differing only in their power supplies. The 6AA/0 was suitable for the 200-250V AC mains, which was used in most parts of the UK.

The 6AA/1 was more flexible and could be powered from the 100-110V AC or 200-250V AC mains. This made the 6AA/1 suitable for use in areas that were still on 110V AC.

 More information

  
Click to see more

Frequency Changer No. 6AC
Introduced in January 1944, this unit is similar to the 6AA, but had a more flexible power supply, that could be configured for the 100-110V AC or 200-250V AC mains, but also for 12V DC.

The 12V DC input uses a vibrator circuit, and is intended for situations where the mains power is failing or unavailable. At least three variants of this model were made, indentified by a numeric suffix to the model number (e.g. 6AC/3).

 More information

  
Frequency Changer No. 6AC/3 with open lid

Privacy Set No. 7, 7A   —  wanted
Introduced in November 1957 for use on CB/ Auto and LB systems. It is housed in a metal enclosure and was known as Privacy Set rather than Frequency Changer.

This version was built around VC138 penthode valves — equivalent to the EF91 — and offers improved performance and security. It has a modular construction, which made it much more service friendly in case of a fault or repair.

 More information

  
Interior of a Privacy Set No. 7A.

Privacy Set No. 8, 8A, 9, 9A
Privacy Set No. 8 was the first model that was fully transistorised. It first appears in GPO diagrams from 1962 and was designed for use on CB/Auto systems only. Like Privacy Set 7, it was also available from commercial parties. It is arguably the most wide-spread post-war model.

Models 8, 9 and 9A are basically the same device with small production and circuit changes.

 More information

  
Privacy Set No. 8 (TE 62/1) with removed cover




Auxiliary equipment used with the scrambler
Auxiliary equipment used with the scrambler


Accessories
Unit Auxiliary Apparatus   SA5050
The SA5050 was a relay unit, that was introduced in March 1944. It allows up to three SA5063/0 voice terminals, or up to three sets of SA5063/1 terminals wired in parallel, to be connected to a single Frequency Changer. Depressing the ENGAGE FOR SECRET (HOLD SCRAMBLER) on one of the SA5063 terminals, causes the SA5050 to disable the other terminals for the duration of the call.   
Relay unit SA5050. Photograph via Andy Grant [1].

Indicator   401CN
In order to inform the other users on the internal network that the Frequency Changer was in use by someone else and was therefore unavailable to you, Indicators No. 401CN were mounted in the vicinity of the SA5063/1 voice terminals. No. 401CN is a so-called dolls eye status indicator [1].   

Connection box BT20/8
...   

Connection box BT No. 6
...   




Transparent telephone set and Privacy Set No. 8

Secraphone
From 1957 onwards, the Scrambler was no longer known as Frequency Changer but as Privacy Set. Also, from the early 1950s onwards, the devices were no longer made by the GPO but by external parties, such as TMC who marketed the system as Secraphone. The image above shows a Privacy Set No. 8 with a transparent voice terminal that was made for demonstration purposes. In SECRET-mode, the microphone and speaker of the handset are routed directly to the Privacy Set, via a 6- or 7-wire cable. The image below shows a TMC advert for the Secraphone [7].

Transparent telephone set and Privacy Set No. 8
Transparent telephone in front of Privacy Set No. 8
Privacy Set No. 8 with cover removed
Seen from the left
Front view
Tranasparent series 300 TMC  telephone set
Three push-buttons
Wiring
A
×
A
1 / 8
Transparent telephone set and Privacy Set No. 8
A
2 / 8
Transparent telephone in front of Privacy Set No. 8
A
3 / 8
Privacy Set No. 8 with cover removed
A
4 / 8
Seen from the left
A
5 / 8
Front view
A
6 / 8
Tranasparent series 300 TMC  telephone set
A
7 / 8
Three push-buttons
A
8 / 8
Wiring

Advert for the Secraphone by Telephone Manufacturing Co Ltd. (TMC). Via Sam Hallas [6].

Real or fake?
Genuine GPO No. 394 telephone sets that have been used as part of a Secraphone secure speech setup, are extremely rare and are very difficult to find. Many collectors have created a mockup by taking an existing multi-button 300, swapping the text shield and adding a green handset to it. If you are a collector and are searching the internet for genuine 394 scrambler phones, there are a few pitfalls to watch out for. Here are some examples of phones that have turned up in the past.

In June 2013, the telephone set shown in the image on the right turned up for sale at Giddings Auctioneers in Leicester (UK). According to the description it is a GPO Telephone No. 394, which had been used as part of a scrambler telephone system, in combination with Security Unit No. 8.

Although this set looks complete and in good condition, we have some reservations about its authenticity. There are strong indications that the set has been put together from spare parts and/or from parts from other telephone sets. First of all, the unit has a dial and, although it is technically possible, this was very uncommon for terminals that were used with the Scrambler.

Secondly, and more seriously, the two buttons, marked NORMAL and SECRET, are labelled the wrong way around. Although various designs of the text label are known, the SECURE button is always on the left and the CLEAR button on the right. This phone therefore could be a fake.

  
Telephone No. 394. Copyright Gildings Auctioneers [11].
Another example of a non-authentic scrambler phone is shown in the image on the right. The item appeared on eBay in February 2016, but in this case, the seller made it very clear in her description, that it was a reproduction [13].

According to the description, it consists of a Telephone set No. 328L of 1962 vintage, with a green reproduction handset that was added at a later date. The scramble/normal plate is genuine and was given to the seller in 1981 when she was an apprentice at GPO/BT, by a former fitter who used to work in the Houses of Parliament.
  
Image of a reproduction Secraphone terminal as sold on eBay in February/March 2016 [14]

On both the above examples, the dials may seem out-of-place, but are necessary if you wanted to operate the Scrambler via a local PABX, like in the second case. If the dials were replaced by a genuine blanking panel No. 3, they would look excellent in any WWII Scrambler Phone display.


YouTube
An excellent documentary about Churchill's Cabinet War Rooms is available on YouTube via the link below. It features a combination of archive film material from the Imperial War Museum's vast collection, and atmospheric dramatisations filmed inside the actual Cabinet War Rooms.




Manufacturers
It is believed that during WWII, Frequency Changers were only made by the General Post Office (GPO), who also developed the unit. The first post-war model to be manufactured by TMC as well was Privacy Set No. 6AC [1]. The later {Privacy Sets were made by the following companies:

  1. All models.
  2. Frequency Changer 6AC, Privacy Set 7, 8/8A and 9/9A.
  3. Privacy Set 8.
  4. Privacy Set 8/8A and 9/9A, albeit possibly with different dimensions.
  5. In the mid-1960s, TMC was taken over by Pye in Cambridge. In 1967, Pye was taken over by Philips.

Connections
BT20/8 Block Terminal
  1. Red
    Microphone/Speaker (common)
  2. Blue
    LINE B (telephone line)
  3. Green
    LINE B (Privacy Set)
  4. White
    Microphone
  5. Orange
    Speaker
  6. Black
    Line A
  7. Grey 1
    Recall (to PABX when present)
  8. Brown
    Ground
  1. The Grey wire was usually identified as SL (Slate).

Glossary
Expressions and abbreviations used on this page:

AC   Alternating Current
Type of current used for most mains networks in the world, typically with an alternating frequency of 50 or 60 Hz. During WWII, most (but not all) of the UK had an AC mains network with a voltage of 110 or 230V. Some regions remained on DC for several years.
BT   (1) British Telecom
Arguably the largest telecom operator of the UK. Previously state-owned and known as the General Post Office (GPO) or the British Post Office (BPO).
BT   (2) Block Terminal
GPO expression for the connection box between a telephone set and the line.
CB   Central Battery system
System in which all the energy needed for transmission and signalling is delivered by the exchange. No local batteries or hand generators are used at the telephone end.  More (off-site)
CBS   Central Battery Signalling system
Similar to a CB system, except that the mircophone is powered locally by a battery at the telephone end. Power for signalling is provided by the exchange as in a CB system. In the UK there were three types of CBS.  More (off-site)
DC   Direct Current
Type of current typically used in cars, but not on the mains network. During WWII, parts of the UK still had a DC mains network, whilst the majority used AC.
GPO   General Post Office
The state-owned post and telecommunications operator in the UK, before it was renamed BT and privitised. The GPO was als known as British Post Office (BPO) and simply as Post Office (PO). It is currently known as British Telecom (BT).  More
LB   Local battery system
System in which a local battery is used for providing the current for the speech circuits.
Magneto   Hand-cranked electrical generator that provides electricity for signalling in an (old) telephone system. In some countries known as inductor, crank ringer, or wake-up unit.
PABX   Private Automatic Branche Exchange PBX
PBX   Private Branche Exchange
Local telephone exchange or switching system, using inside the building of a private organisation, usually connected to the Public Switched Telephone Network (PSTN) via Central Office (CO) lines.
PL   Plessey
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.
TE   TMC (see below)
Manufacturing code used on the body of the telephone sets and also inside, often stamped on the chassis. The manufacturer's code is also cast inside the bakelite body.  More
TMC   Telephone Manufacturing Company
British telephone manufacturer. Also one of the manufacturers of the Privacy Sets. Based in St. mary Cray (Kent, UK) and London [7].  More
Documentation
  1. Circular label Speech on Telephones is Not Secret (PDF)
    Crypto Museum, Reproduction, 10 June 2014.

  2. Original Privacy Set block diagram
    Date unknown. Kindly supplied by Andy Grant [1].

  3. Privacy Set No.8/9 circuit digram, Secraphone Type S2N
    EC526518. TMC London. Date unknown.

  4. Connection diagram SA 5030 to Privacy Set
    Date unknown.
References
  1. Andy Grant, Everthing that you need to know about scramblers but were afraid to ask
    Telecommunications Heritage Journal (THJ), Issue 99, Summer 2017. p. 11—14.
    Reproduced here by kind permission from the author.

  2. Robert Freshwater, 200 Type telephone information
    BOBs Telephone File (website). Retrieved September 2018.

  3. Robert Freshwater, TELEPHONE No. 394
    BOBs Telephone File (website) Retrieved June 2014.

  4. Imperial War Museum (IWM), Photograph of Churchill in Cabinet War Rooms
    Retrieved January 2014. Colour version obtained from [1] - September 2018.

  5. Post Office (GPO). Index to E.I.s 1 on external construction and maintenance
    11 September 1959. Retrieved January 2015.

  6. Post Office (GPO). Index to E.I.s 1 on external construction and maintenance...
    4 April 1962. Retrieved January 2015.

  7. Telephone Manufacturing Co Ltd. (TMC), Advert for Secraphone
    Date unknown, but believed to be late 1950s. Retrieved January 2014.
    Via Sam Hallas, website.

  8. PO 2 Telecomms Headquarters, Automatic and Subscribers Privacy Equipment...
    ...Telephone No., 740 and Privacy Set No. 8
    Document number N5164, 12 March 1975.

  9. PO 2 Telecomms Headquarters, Automatic and Subscribers Privacy Equipment...
    ...Telephone No., 740 and Privacy Set No. 8
    Document number N5166, 23 September 1974.

  10. POTDD (GPO), Telephone No. 394
    Document number N494, issue A, 25 January 1968. First released 30 September 1937.

  11. Gildings Auctioneers, Images of Privacy Set No. 8 and telephone sets
    18 June 2013. Retrieved January 2014.

  12. Connected Earth, Wartime communications: Black and green secrecy phone...
    Retrieved January 2015.

  13. Pandora Blake,
    GPO Black Bakelite 300 Type Phone with Scrambler button and Green Handset

    eBay seller phased_001, item 301880390186. Retrieved February 2016.

  14. P.O.E.D. 3 , N 620, Labels 252 & 253, for use with telephones with keys
    Fist issued 20 April 1952. Last updated 19 April 1967. Obtained via [3].

  15. P.O.E.D. 3 , N 264, Telephone No. 164 (handset) diagram of connexions
    First issued 25 January 1935. Last updated 5 January 1968. Obtained via [3].

  16. Wikipedia, Churchill War Rooms
    Retrieved February 2016.

  17. Louis Meulstee, Wireless for the Warrior
    Website. Retrieved January 2015.
  1. E.I. = Engineering Instructions.
  2. PO or P.O. = Post Office.
  3. P.O.E.D. = Post Office Engineering Department.

Further information
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Crypto Museum. Created: Tuesday 28 January 2014. Last changed: Tuesday, 10 August 2021 - 15:14 CET.
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