M-5 was a compact valve-based clandestine transmitter, also known as a
spy radio transmitter,
developed shortly before World War II by
Army Depot Company Munkkiniemi near Helsinki (Finland),
and introduced in 1940.
It was commonly used in combination with the
Kynnel M-7 receiver.
The design of the transmitter was later also used
in the M-10 and M-11 radio sets.
Louis Meulstee, Wireless for the Warrior, volume 4
ISBN 0952063-36-0, September 2004.