The indicator groups were a problem. Letters 1 and 5 were discriminants and could not be meddled with so that only 6 letters remained whereas it required 8 letters to give the doubly encyphered 4 letter set-up. The Germans did not solve this problem by increasing the indicator groups to 5 letters each, but instead recyphered only the first 2 letters of the set-up: thus we had throw-ons and could obtain alphabets for the first 2 positions of the Grundstellung only.
The alphabets thus obtained still provided sufficient material for breaking, the difference being that the menu required a 4 wheel bombe and that it was no longer possible to reduce the wheel order by Banburismus: as pointed out earlier, the introduction of a 4th wheel reduced the amount of depth for below the subsistence level of Banburismus.
Breaking by means of alphabets was not, however, the method usually adopted as a more economical crib policy could be pursued. Once the alphabets had been obtained the position of the reflector wheel on all messages was known and, as the clip in this wheel never moved, the true position of the core of this wheel was known: provided therefore that the bombe could be persuaded to keep that wheel motionless in whichever of the 26 positions was used on the message, a crib could be run as a normal 3 wheel job. This was possible on the American bombe and intensive research produced a number of excellent cribs. Breaking of Porpoise on cribs gave little difficulty until the autumn of 1944 when advances in Italy had killed most cribs and the Adriatic and Aegean areas split off into independent keys. On March 1st there was some doubt as to whether all 4 reflector combinations had to be considered or whether Porpoise would stick to Bruno Beta: in fact, like later keys changing to 4 wheels, Porpoise used any of the 4 combinations.
At this time, March 1944, the keys being broken regularly were:
There was some rather half hearted breaking of Turtle, constantly abandoned if bombe time was difficult to come by and various oddments like Sunfish and Seahorse were occasionally broken.
Mention must be made here to a very remarkable Dolphin crib which died at about this time after a life of some 4 months. This crib was known to us as XANT XANT. Somewhere, in the Arctic 3 German trawlers - SCHATZGRAEBER, BASSGEIGER, and KREUZRITTER - were transmitting weather information on a cypher known as L.M.T.