Hey opotable, I agree, this is a very interesting guitar. It is definitely Japanese with an interesting mix of features. The headstock/neck is a Kawai product, the body is associated with Minister guitars. I've seen the pups on Firstman guitars. The knobs are clearly replacements. Not sure about origin of all the plastic around the pups and switches but it is typical of Japanese construction. The tailpiece is also clearly Japanese.
Here is a Minister body that comes close to matching yours. I would speculate that your guitar is a early 70's version of this late 60's instrument.
As you well know, Japanese companies bought parts from each other during this period of guitar making so identifying a specific manufacture is difficult. Based on what I see I would go with Minister, they were a smaller producer and likely turned to Kawai to supply their guitar necks. Kawai produced this neck with block inlays on their models, this one has dots. A small twist to distinguish the two.
Very cool find, I hope you bought it.
s