Aha - the mysterious shield-shaped neckplate! I've never owned this exact guitar but I've been upclose & personal with some of its relatives! Have a look & read of this, your guitar's related to my SG and even more closely to the bass I had in 1978, back when dinosaurs ran things:
http://www.matsumoku.org/ggboard/viewtopic.php?t=2926
I'd hazard a guess that your guitar's about 10 years older than your suggestion of 81 - 85. Also I don't think the "original" pickup's original (although the metal pup ring is) and I think the bridge & tailpiece might be replacements too. I think this would have had gold hardware & covered pups, like my SG. I'll also hazard a guess that the neck on this will be that odd "strip mahogany" laminate, rather than solid timber. Have a close look & see if there's any grain pattern visible through the paint on the neck or head.
As for Matsumoku, I don't know, theirs seem to be the 80s original designs rather than copies. That said, the builder of these early shield-plate copies is still a mystery, and until someone unearths one with a brand with a thoroughly recorded history, they'll probably stay that way.
Jon.