|SCYK||RBXO||Grid Square BE41|
|AZEE||MBGV||31 = BE4131|
|TNFX||QQYY||Course towards South|
Messages were normally of length 1 - 6 four letter groups according to the type of report being made.
Considerable parts of the phrase book and first indicator book had been built up by November 1942 when these documents, together with the second indicator book, were captured from U 559.
Documents captured after victory show that a new edition of the Kurzsignalheft (1944(C)) was being issued with Kenngruppenheft Nr.7 zum Kurzsignalheft 1944 (I) and Kenngruppenheft Nr. 8 (I).
The new code book was in two parts;
i.Satzbuch -- phrases to 4 figure numbers.
ii.Buchgruppenheft -- 4 figure numbers to 4 letter groups of the form a+d = b-c
A Schluesselzahl was added to each number between the use of Parts 1 and 2.
The indicator tables are constructed on the same pattern as Kenngruppenhefts 1, 2, 5 with 3 letter indicators but 4 letter settings.
e) Signatures for Bís, Short E's, WWs etc.
Surface craft and U-Boats using B's WWs etc. were allotted bigram signatures. The series of bigram signatures changed about twice a year, but it was possible to reconstruct it from a skeleton list sent in a long enigma message giving the new signatures for boats at sea or from comparison of Short Signal decodes and their acknowledgements. Interpolation was possible as bigrams were arrayed alphabetically beside (in the case of U-Boats) a numerical list of U-Boats with non-existent boats omitted. Interpolation therefore gave information about which boats were sunk or not yet built. The job of reconstructing this list was done by Naval Section 4.
On June 30, 1943, bigram signatures were replaced by trigram signatures taken from the Geheime Marinefunknamenliste (GFL). The trigrams were not arranged in alphabetical order but at the time trigram signatures were introduced the GFL was in our possession.