I've never encountered NOS meaning Not Original Source before. In my experience it refers to
New
Old
Stock. This is an item that is not new in the sense that it has been recently produced, but rather in a sense that it has never been used or installed. Think of the all the parts that must accumulate at the back of old repair shops or dealerships, that once the appliances they were used to repair are no longer in common use, are no longer pulled out for installation. Usually these will get tossed (he says with sadness) or sold off as NOS when someone is cleaning out their backroom or when a retailer/repairman shuts down. This can be a best case scenario for owners of old stuff needing repairs (things like radio tubes or chrome/stainless trim are rarely impacted with age if stored right). In other cases as with certain types of plastic or rubber, it can be no help as these materials may degrade by just being exposed to the air. In these cases it's better to look for a repop (re-produced) part or an equivalent. I personally don't have enough experience with these seals to know if they hold up over time in storage so I cannot speak to the value of them in NOS form. I'm sure someone else on the board does though.
More info here if you aren't already asleep.