Perhaps a primer in basic terminology is in order...
NOS= New, Old Stock. That means it is was originally produced by the "factory" and was the same part that would/could have been purchased from a Chevy/GM parts department and will display a GM part number. It might actually be several years old, but has never been used.. i.e. New, Old Stock.
Repro=Usually, but not always means that the part has been manufactured under license(certified) from GM/Chevrolet. It will almost NEVER have the original GM part number affixed or stamped into the metal.
Aftermarket=Copies produced by uncertified manufacturers, usually off-shore, and any number affixed or stamped on it bears no relationship to the original or a certified repro.
FWIW, GM part numbers almost always(there are exceptions) use the last number for a left side part as an odd number. Right side parts have an even number.