On my Jap the engine No (it dates it at 1958) is on the timing side crankcase at the very front, but the cases are not a pair, both seem to be different materials, however on a pair of (matched) cankcases that I have, the engine no is on the drive side side case and is in a different (bigger) form of type face.
Perhaps a case has been replaced (probably) at some time.
I have a copy of the J.A.P by Jeff Clew which has engine No. details, if you need the info I can write it out for you.
Sorry not very helpful.
Colin