We attacked Stage 7 without having cracked Stage 6, so we did not know for certain what type of cipher it was. However, a clone of ADFGVX seemed the most likely guess and the length of the cipher text gave a nice indication to support this guess. The text consisted of 7048 characters, and 7048 = 8 . 881. Since 881 is prime, we guessed that the code matrix was eight columns wide.
It was pointed out later that this was not necessarily a correct assumption to make since the initial columns could well be of different lengths. However, even after having solved this stage, we felt that that would probably have been to expect the cipher to be more complicated than it was, an assumption which turned out to be true. We had already made the mistake of overestimating the difficulties when trying to solve Stage 3. So even if we had received this warning beforehand, we would have started out with the simpler version.