The only obscure reference to those call numbers I could find is here http://www.goldenyears.ukf.net/reg_J-3.htm
Looks like 2 were registered at some time, so it's a real aircraft,,

I've googled every possible way I can think of, I'm usually able to find it if its' there,, This one has me stumped

Good luck

Other possibility is that it a Germany or Italian design that was built by Kawanishi


p.s.
Post some pictures if you can,, Thanks