Keep in mind that 'NOS' means New, Old Stock.

Doesn't mean original equipment, or even that it was made or authorized by GM.

For instance, if a parts company happened to have made that replacement part back in '63.. and it was never sold, it would be a 'New, Old Stock' part.

Also remember that it's not uncommon for auto manufacturers to source their parts from multiple vendors, and the parts supplied from one vendor may not be 100% identical to the same part from another vendor.

an idea is only stupid if you think about it rationally.