Before digressing on Porpoise and Shark we left Dolphin with the statement that little of interest happened until the autumn of 1943. This was perhaps a slight exaggeration. Nothing of note happened in 1942 and the first event of significance in 1943 was the move from the old Hut 8 into Block D on February 19th. For sentimental reasons this was a wrench but on all other grounds the improvement was immense as it would soon have become impossible for us to manage in the restricted area of Hut 8. On March 1st a change of bigram tables took place, but this time we were able to take the problem in our stride: more bombes, more traffic and perfected methods made the work much easier and after about 3 hectic weeks we were able to restart Banburismus and continue as before. This time no pinch interfered with our efforts and we were able to complete the amusing process of working out the "Tops" or first half of the bigrams with the help of the reciprocal property of the bigram tables.
At the end of April there was a scare that the Mediterranean U Boats would use a separate key, but the orders were cancelled and Turtle did not come into force until June, when it started its career as the poor relation of Shark. July found us all engaged on the Stecker Knock Out to break the new Shark reflector and August brought 2 new keys, much of the history of both of which should really be written by OP-20-G who were entirely responsible for them in later stages. Both used the throw-on indicator system we have met on Porpoise, but traffic was insufficient in nearly every case for the discovery of alphabets and they had to be broken by running throw-on menus on the bombes.
The history of Seahorse begins far back in 1939. Morris in his history of cryptographic work in Naval Section writes:
"The first high grade Naval Cypher to be broken during the war had been worked upon outside N.S. This was the "Kriegsmarine" or Naval Attache Cypher. The code book had been acquired from the French and the subtractor table was reconstructed by Mr. de Gray and his colleagues. The traffic could be read if there was a depth of 3 signals between Berlin and any one of its correspondents..... After the battle of the River Plate, the German Attache at Buenos Aires acquired the Enigma Machine from the Graf Spee and thus the depth of Kriegsmarine traffic fell below subsistence level. Early in 1940 the whole of it went over to a Machine Cypher which was long afterwards broken and is now known as Seahorse".
The first we heard of Seahorse was in August 1942, when some traffic was sent over from Twinn's Section with the suggestion that it might be on our machine. As its indicators implied a 4-wheel machine and our keys were still in general 3 wheeled, this was rather a surprising phenomenon but the key duly broke with reflector Bruno Beta. By now the traffic was much more restricted than in 1939;