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books of settings (SPRUCHSCHLUESSELSAETZE) were used on each link.
It was usual (though not invariable) for all four machines used on a given fish link to be of the same type. The rule was broken particularly when spare machines were brought into use. For a long time for example on Gurnard
Berlin transmitted and Zagreb received on SZ 42B (psi 1 lim.)
Zagreb transmitted and Berlin received
on SZ 42A (chi 2 lim).
(b) Transmissions.
Tunny operators can transmit to each other either in cipher or in clear according to whether the Tunny machine is switched IN or OUT, and either in HAND or in AUTO. If sending and receiving machines were working simultaneously, transmission is described as DUPLEX, otherwise as SIMPLEX.
After Oct. 1942 the normal routine was somewhat as follows: The operator sits at the keyboard of the sending teleprinter with the printer of the receiving teleprinter directly in front of him. He makes contact with the operator at the other end by hand transmission in clear, and may carry on a brief conversation in Q-code to ensure that conditions are satisfactory for cipher transmission.
Before the Tunny machine is switched in, the operator sets the wheels to the settings opposite the next number in the QKP book and transmits QKP followed by the last 2 figures of the number. Just before switching in he transmits UMUM in clear.
After the machine is switched in, all outgoing transmission is in cipher. Further chat by the operator may be answered in clear, or, if the receiving Tunny is also switched in, in cipher. The text of the operator's chat (clear or cipher) is received on the printer but not preserved.
As soon as the operator is ready to transmit his message (which should have been previously perforated) he switches in the auto transmitter and ceases to operate the keyboard. The message starts with an address and serial number and as it is received it is stuck on a message form by the receiving operator.
The transmission of a complete tape is usually followed by operators' chat in hand and then mixed hand and auto transmission while the sender tries to discover if the message has arrived in comprehensible form, makes any necessary corrections, or retransmits any part of the tape. When the receiver is satisfied, he sends a receipt in clear or cipher according to whether his outgoing Tunny is switched in or not.
After the receipt, the sender may switch off or send another message before resetting. One transmission therefore may contain several serial messages. On the other hand, very long message tapes may be transmitted partly in one QKP and partly in the next, and resetting may also take place during a message if something goes wrong.
(e) Repetition of P.
Hand transmission is by no means continuous and a PAUSE implies that the operator has stopped to think or is waiting for the other operator to reply.
Pauses in auto may also occur. Sometimes two tapes are transmitted without any intervening hand transmission, and there is a pause while the new tape is inserted. More frequently, something goes wrong and auto transmission has to be stopped and restarted. When this happens the tape is moved back so that the last 100 letters are retransmitted. In the decode, therefore, 100 letters or so will be repeated. This repeat of P is known as a GO-BACK.
When the pause is accompanied by the resetting of the wheels and the transmission of a new QKP number, the tape is still set back so that the last 100 letters or so of the P of one transmission are reciphered at the beginning of its successor. This is known as an OVERLAP.
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