Once, at a Super Chevy vendor seminar that A & M Soft Seal was putting on, the rep said that one way to tell "off shore" from "domestic" rubber was in how the wrapper/package looked. The "off shore" items would have a brown or black residue on the inside of the plastic or cellophane wrapper, whereas "domestic" rubber had very little. The Chevy LUV rubber items (i.e., Isuzu) usually came packaged in a clear wrapper. Although they were of current production, by the time they'd gotten over here, the inside of the clear wrapper was so black you could not see clearly through it, typically. That put some credibilty to what the rep said in the 1980s--things might have change since then.
Always good to check out the Brand X events for vendor information and literature, at least back then.