In The Japanese FUJI diplomatic cipher 1941-43 I’ve added the following information under the paragraph ‘Pers Z effort’:
More information is available from the TICOM report DF-31B ‘How J.B. 57 Japanese Letter System Was Solved’, written by the cryptanalysts Annalise Huenke and Hans Rohrbach.
The first break into system JB 57 came through two messages that had the same indicator (meaning they used the same transposition key). Once these were solved the system was identified as a transposed code, using a stencil.
Solution of this indicator led to the decipherment of more messages and dr Kunze (head of the ‘Mathematical Cryptanalytic Subsection’ of Pers Z) was able to use the information recovered in order to solve more message indicators. The inroads made by the solution of indicator groups led to the eventual recovery of the underlying code by the linguistic group and the current exploitation of this traffic.