to 4 wheels in February 1942 we had no 4 wheel M bombes and there did not appear to be any likelihood of our getting any in the near future. Various forms(which I shall make no attempt to discuss) of 4 wheel bombe were discussed but it appears to have been decided fairly early that the 4 wheel bombe would have to be like the 3 wheeler but that it would have to work much faster. This clearly was an immense problem and it gave a great deal of trouble. The final 4 wheel bombe had its fast wheel turning at some 10,000 revolutions per minute, leaving about 1½/1000 seconds for the complete examination of each machine position and for that split second every contact on every wheel had to be precisely in the right position or the job would probably be missed.
Work on the High Speed Machine was begun by Wynne-Williams late in 1941 and some 9 months later Keen, who was largely responsible for the 3 wheel machine, started work independently, as by this time the success of the Wynne-Williams 'Cobra’ was very much in doubt. The Cobras were in fact eventually brought into action and proved very useful: production was, however, stopped after the delivery of 12 machines as it was obvious that the Keen machine was both more efficient and more adaptable. In early 1942 we were far from possessing any High Speed machine of any sort - the first came into action in March 1943 - and we were reduced first to exasperated inaction and then able to start breaking with 4 wheel bombes by means of Short Signal menus. The Short Signal period began in December 1943 and the following chapter on Short Signals has been written independently by Ashcroft who was in charge of research into this subject.
1. i. Description of Short Signals.
In addition to the normal type of Enigma signal, which has already been described, the German Navy, from 1940 onwards, used certain special types of abbreviated signal which were cyphered on the same keys but with two distinguishing characteristics.
a. Message settings were taken from an unrecyphered indicator table, the indicator appearing in clear in the preamble of the signal.
b. Messages before cyphering consisted of code words or groups taken from a book.
Fulfilling this definition were 4 main categories of Short Signal, each with its own book and indicator tables. There were