Per [MENTION=20650]T-Mar[/MENTION]'s Asian Serial Number Guide, not all Nishiki frames were made in Japan. Some of the (presumably) later ones were built in Taiwan by Dodsun, Fairly, and Giant. This transition appears to have occurred in the late 1980s - and I believe that's also about when TIG-welding started to come into its own re: frame production. The fact that the OP indicates (post 28) that all of his frame joints except the chainstays appear to be welded vice lugged and brazed argues for this being a strong possibility; ditto the vertical dropouts (see photos).

I'm wondering if perhaps the BB shell or (more likely) chainstays were replaced at some point. I'd expect those joints to be welded too if the other joints were welded. But I guess it's also possible that the OP's frame was from a transitional period where some joints were brazed and the rest were welded.