Pro/Stock was an old brand made specifically for PAW and sold only through them at first. They made everything from intakes to valve covers to camshafts. You could probably email the guy at PAW and he can tell you. Although PAW now only specializes in early Hemi stuff, I am sure he can help you because that was their exclusive brand.
It's probably a good intake, I suspect. Back when they were made, outsourcing was not as bad of a problem as it is now, so most assuredly, it was cast right here in the US. Probably a Holley or Edelbrock cast piece for Pro/Stock. I bet it's a good piece.
I have one also(for sale)these are like a copy of the TM5 intake.It is a single plane intake,if you didn,t know?They flow pretty good too!Here,s a pic of the 2 to compare. View attachment Parts to sell 012.jpg
It's closer to a Holley intake IMO. Also, during the period they were being made, a EGR valve was cast into it for that just in case it needs to appear or actually pass a test.
That intake should be in a RPM range like the Torker, 2500 - 6500 rpm's. And IMO, work better than the Torker(s).
A 600 would be perfect for a 273. If it was jetted up for a bigger cube motor you could always jet it back down. It may be good where it's at. The intake may be too much for a 273 though. Check the port match. 273 heads had small ports. toolmanmike