Yes, T-mar has a great thread on Nishiki serial numbers. Here is the thread, the first post contains the serial key.

My guess is yours is a mid 70's model like mine was. Does yours have a cro-moly frame, or is it hi-ten? Mine had a hi-ten frame unfortunately but still had nice forged drop outs and interesting chrome. The decals seemed to be pretty crappy and the paint didn't seem much better.