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Rotor Enigma G → Z → ← D
Zählwerksmaschine A28 · 1928
Enigma model A28, also known as Zählwerksmaschine (counter machine) [4],
is an Enigma cipher machine, developed in 1928 by
Chiffriermaschinen AG (ChiMaAG) in Berlin (Germany).
It was the first glowlamp Enigma to use an alternative rotor stepping
mechanism, driven by cog-wheels and pinions rather than
the more common ratchet and pawl arrangment. The three cipher rotors have
17, 15 and 11 turnover notches respectively and the reflector (UKW)
moves during encipherment. The machine is also known by its designator Ch.15.
It was succeeded in 1931 by Enigma G (G31).
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The machine is housed in a wooden transit case with a hinged top lid,
and has roughly the same form factor as the Enigma K.
Inside the machine are four rotors, of which the leftmost one is the reflector
(German: Umkehrwalze, or UKW). All rotors have letters on their circumference.
Each of the four rotors can be set to a given position.
This Enigma model has several features that are not available on earlier
machines like Enigma K and Enigma I.
It has a counter
that increments with each key press, and a stepping
mechanism that is driven by firmly coupled cog-wheels.
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As a result, the mechanism can be wound back and forth any number of steps
without loosing the synchronism.
For manual operation of the stepping mechanism,
a crank — stowed in the case lid — can be
inserted into a hole to the right of the rotors.
In addition, the cipher rotors have 17, 15, and 11 turnover
notches respectively, which causes a more frequent turnover of the rotors
than on regular Enigma machines.
Note that on this model the UKW moves during encipherment.
The machine shown here – with serial number A865 – was sold to the Royal Dutch Navy,
probably in 1928, as part of a series of six identical machines [6]
two of which have survived.
It is unknown how many Enigma A28 machines were manufactured in total.
The machine was succeeded in 1931 by Enigma G, which is
fully compatible but has a smaller form factor and smaller rotors.
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The diagram below provides an overview of the features of the Zählwerk
Enigma A28. The unit is housed in an
oak wooden transit case that measures
307 × 282 × 160 mm and weights 11.3 kg (case included).
The hinged case lid holds 10 spare light bulbs, a crank
and optionally a green filter. It also holds a couple of metal text shields,
the serial number and the acceptance stamps.
The machine comprises a keyboard with 26 keys (A-Z),
a lamp panel with 26 letters (A-Z), a
rotor assembly,
a gear-locking lever (EIN/AUS),
a power selector and two terminals
for connecting an external power source.
In most cases, the device was powered by an
internal 4.5V battery.
Each of the four rotors has a thumbwheel that
protrudes the top lid.
The leftmost rotor is the reflector or Umkehrwalze (UKW), whilst
the rightmost three are the actual cipher rotors.
The rotors and the UKW can all be set to any
of 26 starting positions, and they all move during encipherment.
The current position of each rotor is visible through a window
to the left of each thumbwheel.
On this model, a 4-digit counter is present near the
left edge. It increments each time a key is pressed and allows the number of
characters to be counted.
The crank is not intended for resetting the counter but – in combination
with the counter – for the correction of mistakes.
There are three metal shields inside the case lid. From top to bottom:
a shield that reads: Made in Germany (in English), an oval
shield with the Enigma logo, and a rectangular shield with the name
and address of the manufacturer — Chiffriermaschinen Aktiengesellschaft
(ChiMaAG). 1
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To the right of the rotors is a rotary switch
that is used as the power
selector. It has the following four settings: 1 Bright (hell), 2 Dark (dkl, dunkel), 3 Off (aus) and 4 External (Sammler). Note that in the leftmost
setting (hell) the lamps are over-powered, as the internal battery delivers
4.5V, whilst the lamps are made for just 3.5V. This was done for two reasons:
to overcome the contact resistance of the rotors and to allow the machine
to be operated in bright daylight.
The latter reduces the lifetime of the lamps however, which is why
10 spares were supplied in the case lid.
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Most machines were supplied with a dark-green filter that could be placed
over the lamp panel to increase the contrast 2 when operating the machine in
bright daylight. The filter is held in place by two hold-down clips at the
sides. When unused, the filter could be stowed inside the top lid.
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Differences with earlier machines
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- Cog-wheel driven stepping mechanism
- Coupling lever
- Counter
- Crank (for corrections)
- No double stepping anomaly
- Irregular stepping of the rotors
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- Driven UKW
- Digits above lamp panel
- Digits on upper row of keys
- Wrinkle painted enclosure
- Notch ring attached to letter ring
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Differences with the 1927 prototype
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- Counter with 4 digits (rather than 3)
- Multiple notches on the rotors (11, 15, 17) rather than just 1
- Irregular stepping of the rotors
- Different type of crank
- Green contrast filter
- Driven UKW
- Notch ring attached to letter ring
- Aluminium power knob (rather than Paxolin)
- Power knob and terminals fitted to top lid (rather than protruding it)
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In 1934, the company name – Chiffiermaschinen AG – was changed to
Heimsoeth und Rinke.
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Contrary to popular believe, the green filter was not used for dimming
the light at night, as that could be achieved with the setting Dark (dkl)
of the power selector.
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B
The rotors of the Zählwerk Enigma A28 have the same outer dimensions
as the rotors of a regular machine, such as the Enigma I and
Enigma K, but they are not interchangeable. A regular Enigma
rotor is driven by a
ratchet wheel at its right side,
whilst a Zählwerk rotor
has a cog-wheel with 52 teeth in that position. Likewise, the left side
of a regular rotor has a ring with
one or two turn-over notches,
whereas a Zählwerk rotor has a
cog-wheel of which some of the teeth are missing.
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Regular Enigma rotors (left) and the rotors of Zählwerk Enigma A28 (right)
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When inside the machine, the three rotors and
the UKW are firmly coupled by means of small pinion wheels
that are located behind the rotors.
Once coupled, the rotors can no longer be moved manually. In order to
alter the basic setting of the machine, the rotors first have to
be decoupled. This is done by setting the
coupling lever (EIN/AUS)
at the far left to AUS.
Most machines were supplied with just three rotors that could be installed on
the spindle in six different orders (3 x 2 x 1).
These rotors (I, II and III) had 17, 15 and 11 notches 1 respectively.
The positions of these notches are identical on all surviving machines,
regardless of their wiring and regardless of the customer.
It is known that some machines were supplied with more than three rotors.
For example, the Hungarian Enigma G31 machines (Ch.15b)
came with five rotors.
It is very likely that Zählwerk Enigma A28 was originally intended for
commercial use, as the wiring of the ETW, UKW and in some cases the wiring
of the rotors as well, was identical to the wiring of the
commerial machines.
Sometimes the rotors were rewired by the customer, but in most
cases the wiring of the UKW was left unchanged.
The Abwehr ordered unwired rotors [6].
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For analogy with a regular Enigma, we use the term notches to
indicate a turnover position. In reality however, it is the presence
of a pair of teeth on an Enigma A28 rotor that causes a turnover.
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When our Zählwerk Enigma (A865) was found, the UKW and all cipher rotors had
the same wiring as commercial Enigma K. This confirms that this machine
and its successor – Enigma G31 – were actually commercial machines.
Nevertheless, they were sold to foreign military customers and to the
German intelligence service — the Abwehr.
It is known that some customers changed the wiring of the rotors, but in this
case, the Royal Dutch Navy left the wiring as it was supplied.
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Rotor
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ABCDEFGHIJKLMNOPQRSTUVWXYZ
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Notch
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Turnover
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#
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ETW
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QWERTZUIOASDFGHJKPYXCVBNML
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I
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LPGSZMHAEOQKVXRFYBUTNICJDW
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ACDEHIJKMNOQSTWXY
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SUVWZABCEFGIKLOPQ
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17
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II
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SLVGBTFXJQOHEWIRZYAMKPCNDU
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ABDGHIKLNOPSUVY
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STVYZACDFGHKMNQ
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15
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III
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CJGDPSHKTURAWZXFMYNQOBVLIE
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CEFIMNPSUVZ
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UWXAEFHKMNR
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11
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UKW
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IMETCGFRAYSQBZXWLHKDVUPOJN
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Around 1926, the Enigma manufacturer — Chiffriermaschinen AG —
started the development of a series of new machines, all based on the chassis
of commercial Enigma D (A26).
For the Reichswehr (the predecessor of the Wehrmacht),
they developed the Enigma I and for various
other civil and military customers the
Enigma A27 (later also known as Enigma K)
was introduced.
At the same time, development was started of an improved
machine that was described as follows: [5]
Glühlampen-Chiffriermaschine "ENIGMA"
mit Zählwerk
und zwangläufiger
Kupplung der Chiffrierwalzen.
Translated: Lamp-Enigma with counter and coupled cipher rotors.
It has a cog-wheel driven rotor-turnover mechanism that features
irregular stepping, making it cryptographically stronger than
the regularly stepping Enigma D and K.
Several models were based on this new concept.
All these machines have a counter that registers the number of key-presses,
which is why they are also known as Zählwerksmachine (counter machine)
or Zählwerk Enigma (counter Enigma).
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After the first prototypes in 1927,
the first machine with this mechanism was released in 1928
as the Zählwerk Enigma A28. Two versions of this machine
exist (Mark 1 and Mark 2), the latter of which is featured on this page.
It has the same form factor as a regular Enigma K.
A few years later, around 1930, a
numbers-only version of the machine
was released as the Z30.
Enigma A28 eventually evolved into the smaller Enigma G
(or model G31), which was released in 1931.
The two Enigma models are interoperable.
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All that changed when, in 2007, we discovered an original Zählwerk Enigma
that matches the patent descriptions of 1928. The machine, with serial
number A865, was probably manufactured in 1928 and was sold to the
Royal Dutch Navy.
The image on the right shows the rotor assembly of that machine, which shows
great resemblance to this drawing from
US Patent DE534947.
Note the position of the counter and the crank.
We later learned that this model was developed in 1928 and that
it is the predecessor of the Enigma G. It is known as
Enigma A28 (Ch.15).
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Several years later, Enigma researcher Frode Weierud discovered that this
machine was part of a batch of six A28 machines that were sold to the
Royal Dutch Navy, probably in 1928 [6]. It was delivered through
N.V. Voorheen Ruhaak in Den Haag (The Hague) — the Dutch representative
of ChiMaAG. Between 1931 and 1940 the Dutch Navy purchased several large
batches of Enigma G, which is compatible with the A28.
In total, the Dutch Navy had at least 93 counter machines.
As far as we know, there are only three
surviving Enigma A28 machines
worldwide.
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Zählwerk Enigma (counter Enigma) is based on the design of the
Enigma D (A26) and Enigma K (A27),
but has additional features and improvements. Most of these
extras are described in two German patents:
DE534947 [1]
and DE579555 [2].
The most important difference with earlier models is the way
in which the rotors advance when a key is pressed.
In the earlier Enigma K, and also in the later Enigma I,
the rotors are advanced by means of pawls, ratchets and notches.
As a result, a rotor can only step forward.
Under certain conditions, the middle rotor
can make an additional step on two successive key presses.
This is known as the double stepping anomaly [8].
This stepping behaviour is also known as Enigma stepping.
With Zählwerk Enigma however, the rotors are advanced by a cog-wheel driven
mechanism that does not suffer from the double stepping anomaly.
In addition, the number of notches on each rotor has been increased.
It is different for each rotor, does not share a common factor and cannot
divide 26. By using 17, 15 and 11 notches respectively,
the longest possible cipher period is obtained.
This stepping behaviour is also known as irregular stepping.
Another difference with Enigma K is that the reflector (UKW)
can not only be set to any of 26 positions,
but is also moved during encipherment. This is known as
a movable or driven UKW.
The three cipher rotors are mounted on a removable spindle,
just like with other glowlamp Enigma machines, but the movable UKW
is fitted permanently. It cannot be removed easily.
Each rotor has a full cog-wheel with 52 teeth attached to its
right side. On the left side is another cog-wheel with the same
spacing, but with a number of teeth missing.
The presence of a pair of teeth is equivalent to a notch on an
regular Enigma rotor.
When the rotors are engaged, they are firmly coupled by means of
4 small pinion wheels with teeth of alternating length.
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Left and right side of an Enigma A28 rotor
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As a result, the entire mechanism can be stepped forward and backward,
without losing the relation between the position of the rotors. A crank
can be inserted into a hole in the body of the machine, allowing the
mechanism to be wound back to the desired position.
This was used to correct mistakes,
but could theoretically also be used as part of the cryptographic procedure.
The rotors of the initial Zählwerk machine — Enigma A28 — have the
same diameter as the rotors of a regular Enigma, such as Enigma K.
With the later G31 model however,
smaller rotors are used, as illustrated below.
In order to accomodate the spring-loaded contacts,
they are arranged in a zig-zag pattern, whilst the contact pads at the other
side have an oval or tear-drop shape.
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The rotors of the Zählwerk Enigma model A28 (left) and the Enigma model G31 (right)
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German Patent DE534947 describes the basic
properties of the cog-wheel driven stepping mechanism, whereas Patent
DE579555 describes the use of multiple
notches on the individual rotors, which leads to a more frequent stepping
of all three rotors and the UKW [1][2].
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The interior of the machine can be accessed by releasing the two
knurled bolts
just behind the keyboard, and raising the hinged lid. This exposes the
light bulbs, the rotor assembly and the
battery compartment. At first sight
the rotor assembly may look similar to the rotor assembly of the Enigma K:
it has a settable reflector (UKW) and three moving cipher rotors with
letters (A-Z).
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When looking closer however, it becomes clear that the
rotor stepping mechanism
is actually quite different. Each rotor has a
cog-wheel with 52 teeth
at its right side, and an incomplete cog-wheel (of which teeth are missing)
at its left side. Around the entry disc (ETW) is a revolving cylinder
with a ratchet wheel and a cog-wheel.
This driving cog-wheel and the rotors are firmly coupled by means of
small pinion wheels
with theeth of alternating length.
They are visible in the image on the right, in which the
rotors have been removed from the stepping mechanism.
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On each key-press, the ratchet wheel is advanced by one step
and the rightmost rotor moves to the next letter.
Depending on the presence or absence of teeth on the left side of each
rotor, one or more rotors will make a step. Note that the UKW can also
be moved by the adjacent rotor.
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To the right of the rotor stack, just in front of the ETW, is
a cog-wheel assembly that allows a
crank to be installed
through a hole in the top lid.
For the correction of errors, the crank can be used to turn the rotors
one or more steps back and forth in full synchronism.
This feature is not available on any regular (non-counter) Enigma.
Via a long shaft, the cog-wheel assembly also drives a
4-digit counter at the left.
This counter increments each time a key is pressed
and cannot be reset.
The operator therefore has to write down the value at the start
of each message.
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After placing the three cipher rotors on the spindle
in the desired order,
the rotor stack can be installed in between the entry disc (ETW) and the
reflector (UKW). The UKW is then pushed to the right and locked in place by pulling the UKW release lever towards the front of the machine.
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Next, the pionion wheels must be coupled to the cipher rotors by placing the
coupling lever in the frontmost position (EIN).
This is done by pulling the lever upward, moving it to the front and then
releasing it.
It is good practice to move all rotors (and the UKW) by several steps to
ensure a good coupling of the pinions. The machine is now ready for operation.
Note that, when the lid is closed, the coupling must be disenaged (AUS)
before the basic rotor settings can be altered.
Unlike the rotors, the UKW cannot be removed
without partially disassembling the machine.
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The rest of the machine — the keyboard, the lamp panel and the battery
compartment — are very similar to those of the Enigma K and
Enigma I. The principle of the cog-wheel driven stepping mechanism
was later also used in the Enigma Z30 Mark 2 and the
Enigma G (G31).
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Early Zählwerk Enigma
1927
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This machine was built before the patents of its design were
actually filed and should probably be regarded as a pre-production
series. The counter was placed to the right of the rotors
and each rotor had a single turnover notch, which means that it
featured regular stepping [3].
Furthermore, the UKW is not driven in this version as the
corresponding pinion wheel is missing.
Only a few of these machines were ever made. The only known
surviving one is held in the collection of the
Swedish FRA [3].
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Zählwerk Enigma, model A28
1928
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This machine is identical to the drawings in German patents
DE534947
and DE579555.
The rotors are coupled by cog-wheels and pinions, and
each rotor has a different number of turnover notches — 11, 15 and 17 —
all relative primes of 26. This maximises the
cipher period.
On this model, the counter is located to the left
of the rotors. The machine described on this page
is of this type.
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Enigma Z, model Z30 Mark 2
1930
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The Enigma Z is a numbers-only version of the Enigma.
It was intended for the encryption of numeric data, such as
weather reports. It is known that two versions of this machine
exist, which we have named Mark 1 and Mark 2.
Although not really a Zählwerk Enigma — it does not have a counter —
the Mark 2 version of this machine
is clearly based on the same cog-wheel driven stepping mechanism.
➤ More information
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In 1931, a smaller version of the Zählwerk Enigma was introduced.
These machines not only had a smaller body, but were also equipped
with smaller rotors.
Furthermore the battery compartment was replaced by a smaller one,
and the counter was moved (again) to the right.
These machines all had serial numbers starting with the letter 'G',
which is why they are commonly known as Enigma G.
They are also known as Abwehr Enigma or model G31.
➤ Further information
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Device Rotor cipher machine Brand Enigma Model A28 Designator Ch.15 Serial number A865, see below Manufacturer ChiMaAG Country Germany Users Dutch Navy, Chile, Swedish Army and Navy Rotors 3 Turnovers 17, 15 and 11 for rotors I, II and III respectively Reflector Settable and movable (driven) Wiring see above Stepping Irregular (cog-wheel gear) Plugboard No Extras Crank, Green filter Dimensions 307 × 282 × 160 mm Weight 11.3 kg (wooden transit case included)
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A350 1 1927 Swedish Army A351 1 1927 Swedish Army
A823 1928 Dutch Navy A825 1928 Dutch Navy A826 1928 Chile A829 1928 Chile A842 1928 Chile A843 1928 Chile
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A845 1928 Dutch Navy A853 1929 Swedish Navy A854 1929 Swedish Navy A855 1929 Swedish Navy A856 1928 Dutch Navy A858 1928 Dutch Navy A859 1928 Chile A860 1928 Chile A865 1928 Dutch Navy
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FRA, Sweden ➤ Photo A825 Beeld en Geluid, Den Haag (Netherlands) A865 Crypto Museum, Netherlands
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This is an early prototype with a single turnover notch per rotor.
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User manual [A] kindly provided by FRA Sweden
[3].
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- German Patent DE534947 (9 November 1928)
Patent for the cog-wheel driven rotor turnover mechanism and the Ringstellung.
- German Patent DE579555 (17 November 1928)
Patent covering multiple notches on the rotors fixed to the index ring.
- FRA, Personal correspondence
Swedish National Defence Radio Establishment (FRA).
May 2009.
- Chiffriermaschinen AG, Gebrauchsanweising für die Zählwerksmaschine Modell A 28
Enigma A28 User Manual (German). Date unknown, probably 1928. 1
- Chiffriermaschinen Aktiengesellschaft, Herrn Direktor Walter Edström
Offering for Schreibende Enigma and Glühlampenmaschine mit Zählwerk (German).
16 September 1929. Crypto Museum #300304. 2
- Frode Weierud, Personal correspondence
Crypto Museum, May 2009.
- David Kenyon and Frode Weierud, Enigma G: The Counter Enigma
Bletchley Park and Crypto Cellar Research, 5 May 2019. Updated 15 June 2019.
- David Hamer: Actions involved in the 'double stepping' of the middle rotor
Cryptologia, January 1997, Volume XX, Number 1. 3
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User manual [5] kindly supply by FRA Sweden
[3].
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Document kindly supplied by Frode Weierud [6].
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Reproduced here by kind permission from the author.
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© Crypto Museum. Created: Monday 21 May 2012. Last changed: Tuesday, 04 July 2023 - 20:26 CET.
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![First version of the counter machine with serial number A351. Photograph reproduced here by kind permission [3].](img/A351_17_large.jpg)
