The photograph shows a faithful replica of a device used by Finnish forces during the Continuation War against the Soviet Union in the early 1940s.  The device, whose name "ALLU" derives from that of its inventor, the military commander Alvar Ahonen. It  is an extension of the system first devised by Leon Battista Alberti.

Encipherment proceeds by sectors and in groups of five.  The inner disk is rotated so that the metal latch is positioned over a pre-agreed starting sector (the number of each sector is stamped onto the periphery of the aluminum base disk.  In the photograph, this is sector 11).  Plaintext characters are located in the fixed outer disk and the ciphertext  found at the corresponding position on the inner disk, but progressing from the outer to the inner of the five concentric mixed alphabets.  For example, in this setting,  TALVI would be enciphered THDGA.  The two black sectors in the outer disk can be used to indicate a switch between letters and numerals/common words.  After each group of five encipherments, the inner disk is progressed one sector and the process continued.

The Continuation War, also known as the Second Soviet-Finnish War, was a conflict fought by Finland and Nazi Germany against the Soviet Union from 1941 to 1944, as part of World War II.

ALLU cipher disk. Click on image to enlarge. (Photo by Michael Smith)

Contributor:

1) Michael Smith <msthylacine(at)gmail.com>

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April 23/23