A good starting point would be:
PRO record HW 50/13/2. B Machine (Japanese Diplomatic) Anglo/US Cooperation. Notes on the US release to Britain of their greatest secret, the Japanese high-grade diplomatic cipher machine Purple pencil notes by Frank Birch 1st February 1942 – 2nd May 1945.
The significance of the Japanese connection is the German/ Japanese Alliance. Messages sent from the Japanese Embassy in Berlin by Oshima Hiroshi, Japan's ambassador to Berlin during World War II, had been fully decoded by the Americans and sent off to Washington, providing a source of information for the Allies on German activities. These included the Ardennes offensive, and its build up.
For those interested in such matters Charles Whiting gives an interesting account of the "Special Arrangements" between the UK decoders and their American counterparts, with "Magic". Further reading can be found in "The Field Marshalls Revenge".
Jack I'm dissapointed that you should doubt me.