mentioned by the prisomer had already taken place we found that about 70% of these cribs must have been right. Further 'crash analyses' were made for other periods up to Aug 1939, all with fairly favourable results. At the same time there had been some changes in the machine, known to have taken place because of the corresponding changes in the machine used by the army and air whose traffic had been read. In the summer of 1937 the Umkehrwalze had been changed from A to B, in Dec 1938 two new wheels IV and V had been introduced. After the beginning of the war (Sept 1939) the FORTYWEEPY messages were no longer traceable, because there were no more call signs. However there had been some traffic of this kind at various times during manoeuvres and crises since the occupation of Austria, and there were a few days where there was both traffic with and without call signs. We hoped that we might be able to find the keys for some such days and so to find the kind of thing that was said in the traffic without call signs. There seemed to be some doubt as to the feasibility of this plan, as the call signs traffic on any day was always either the whole of the Baltic traffic or the whole of the non-Baltic traffic, and the Baltic traffic in 1937 used to be on a different key from the rest. Following this programme we found the keys for Nov. 28 1938 and for a number of days near there. The number of Stecker was 6. The wheel order and Ringstellung seemed to remain constant for about a week; at any rate did not change between Nov 24 and Nov 29. The Stecker were not hatted; the same letter was never steckered on two consecutive days. This of course might be extremely valuable. If the traffic had been heavier it would have enabled us to find the keys so long as this lasted, and there were any cribs. Actually we got no further than this, as at this point a good deal of data was
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Back to Turing's Treatise on the Enigma. Chapter VII.