David -
I looked around and found this Nishiki Wiki page, created by Steven Vance.

http://www.stevevance.net/nishiki/Main_Page

He is still gathering information, catalogs, and serial numbers -and he might be interested in yours.

You already learned yours is a Kawamura frame, built for the US market in 1976 ... the serial number decoder is here:

http://www.stevevance.net/nishiki/Se...umber_Database

Of course those 600 bits are later replacements. I don't know what it would have had originally (other than center pull brakes: The cable hanger kind of gives that away.)

I have no idea at all what model it is. There seems to have been a proliferation of models.