Rotors of TUNNY Cryptographic Machine
Although the ENIGMA remains the best know German cryptographic machine of World War II, in the early 1940's the German military introduced several new cryptographic teletypewriters known under the name Geheimschreiber - sometimes translated as "private secretary", sometimes as "secret writer".
These machines offered on-line encryption and decryption, that is plain test could be typed directly into the machine, automatically converted to encrypted text, and sent directly to the transmitter. In addtion to security, these "secret writers" provided the Germans with the ability to encrypt large volumes of test at high speed.
Learning that the Germans had named an early version of these machines SWORDFISH, the British and Americans bestowed nicknames associated with fish on the machines and the communications links in which they were used. The two most famous are TUNNY and STURGEON.
Just as they developed the Bombe to assist decryption of ENIGMA , the British developed data processing to attack the fish family of machine ciphers. (I must add: This was a whale of a job!) This led to the construction of the COLOSSUS which British historian F. H. Hinsley is "justly claimed as a pioneer programmable electronic digital computer."
The 40 (SZ40) when first encountered in 1940 was nicknamed TUNNY by the British - after a fish better known to Americans as TUNA.
The Schlüsselzusatz SZ40, manufactured by the German firm Lorenz, was used by the German Army for high-level communications, generally between Army groups. It provided on-line encryption and decryption and was capable of handling large volumes f traffic at high speed. The TUNNY depended on wheels for encryption and decryption but unlike ENIGMA it did nut substitute letters but insted encrypted elements of the electrically generated Baudot code used in normal telegraphic transmissions.
Source: National Cryptologic Museum 13 February 2009 with some hyperlinks added