A word must be said about 'flapsí. Banburismus hardly ever enabled traffic to be read currently. It was about 28 hours after the traffic had started coming in that Freeborn sent up the catalogue, and then the Banburismus had to be worked on, a crib broken on the bombe and the paired day broken -- all within 20 hours if we were to be current at all. We were in fact quite frequently current for an hour or two, but situations arose when it was urgent to be current as much as possible - these were the Scharnhorst-Gneiaenau 'flaps', the Tirpitz 'flaps', and the P Q convoy 'flaps', the last named being flaps in their own right whether or not the Tirpitz was thought likely to put to sea.
Currency could only be obtained by running all wheel order cribs, the objection to which was that they were extravagant of bombe time. However, bombes were rather more plentiful by this time and the expenditure was usually thought to be justified. Cribs were not usually lacking and mines were laid in the Baltic for the specific purpose of producing Werft reencodements as soon as possible after keys had changed and thus give us a chance of an early break. We disliked these flaps as, if once we were current, there was nothing left for us to do as we only had one key and at these times many hours were wiled away with chess and gossip. There were also modified flaps; a good crib would be run on what were thought then to be the best wheel orders (Wylie wheel orders assumed that none of the wheels used in the previous wheel order was being used on the next day, so only 60 wheel orders had to be run instead of 336) or a crib would be run for wheels 6, 7, and 8 on the right only: for technical reasons the Banburismus was more likely to be difficult and slow in breaking if one of these wheels occurred in the right hand position. The advantage of the modified flap technique was that fairly frequently we were able to be current without any very great expenditure of bombe time.
There is a lamentable story behind the original breaking of Porpoise, undoubtedly the worst episode in the history of the Section.
Porpoise used the throw-on indicating system which we mentioned briefly in connection with early Enigma. By this system a trigram of the operator's own choosing was encyphered twice at the Grundstellung and the resulting 6 letters became letters 2, 3, 4, 6, 7, 8, of the indicator groups: letters 1 and 5 indicated the key in accordance with Kenngruppenverfahren Sued.