I have been trying to duplicate some threads on mid 1950's vintage Japanese model engines and find that the threads do not fit into current Japanese thread standards. In particular one size is almost exactly a 5-40 thread while another is just a few thousands larger in major diameter than a 12-32 thread. For shallow holes the 5-40 and 12-32 taps work but for deeply taped holes binding occurs. I was hopeful that someone knows where one can obtain info and preferably suitable taps. The threads are definitely not metric or an early British standard.
Don't know, Bob. I am sure you have done some basic research, but if not, look at these links below. The second link welcomes enquiries:
J.I.S. Information and Online Shopping
Red to red and black to black, or it's ashes to ashes and dust to dust.
this may help. I scanned the first pages of the metric and standard threads from the machinery handbook.
I can email you the scans if you would like. probably be easier to read that way.. PM me with your email if you want them.