Hi krait,
You're correct, there is a difference between a B-west made receiver and a rifle imported to B-west. As long as the rifle came from the Norinco/Polytech factory(s) you're in the clear, I would kick myself in the ass if I paid $600 for a B-west made receiver and the opportunity came knocking not long ago in fact but I declined based on the lack of heat treating with the rifle.
Do you see a 66 or 386 on the rifle? They are generally in the middle of a triangle or oval on the left side of the receiver/trunnion. the 386 factory is the polytech factory where the Polytech Legends were made and 66 is a known Norinco factory though both companies are closely associated with each other. There could be other numbers as well if im not mistaken, like yourself, I only know what Ive read.
Most of these rifles are clearly stamped with Norinco or Polytech, however I am more familiar with the post ban Norinco rifles than I am with the pre-bans which looks to be what you've got there.
Do you know if the serial numbers match on the gas tube as well? I generally want my serial numbers to match on the tube, trunnion, bolt, bolt carrier etc. I passed up a rifle for a good price recently because of this very problem. It doesnt really hurt the function of the rifle, but it does hurt the overall value. It was in rough shape anyway.
You've got something a lot better than a beater rifle I think, I would take care of it if I was you. Shoot the hell out of it of course, but keep it stored in a nice place and keep it relatively clean even though with these rifles there is plenty of room in that department. If you're looking for a trunk/beater rifle, throw $350 on a cheap, used Romanian Century build imo. The Chinese rifles hold value and with the 1989 and 94 assault weapon bans, a lot of people find them desirable.