[OG68]

To me, NOS means that the part was made during or within a few years of the model year. NOS= New OLD Stock
For those parts that GM has made for 20 or 30 years, I try to go by the packaging design of the box/bag/wrapper as GM has often changed that design and you can get some sort of an idea when the part was made. But that doesn't always work either. Sellers often mention that "box not included" or depending upon the honesty of the seller whether the part is actually period correct NOS, current production NOS that you can still buy at a GM dealer or a reproduction. There are a few sellers of NOS parts that I trust and have had good luck with.

And you have to be careful of non-GM aftermarket parts that were made during the time. Some after market manufacturers would copy the design of the GM / Delco parts and / or packaging down to the same lettering font but leave out trademark design elements in the hopes that the consumer wouldn't notice. It happened then, happens now. But some sellers (Ebay) will sell these parts as GM NOS.
It's all a crapshoot and a learning game. Once you learn who to trust it gets easier, but not fool proof.

AFAIK, Ames, Parts Place, Inline Tube, and OPG are the major manufacturers of reproduction Pontiac parts. And they wholesale parts to each other. Everyone knows how they advertise and how they deal with the customer. I applaud those vendors who are attempting to reproduce authentic parts. I can understand the up-front costs to do so are high and they need to consolidate similar part numbers in order to try and make a profit. But at least they should say that they do. I suppose you can say the end seller should let the customer know the differences and I think Ames is pretty good at it as opposed to OPG where everything they sell is "a Perfect Fit". But it often falls upon the customer to determine what is good enough for them.