For us,
It means Pontiac parts with a part number right out of the parts book.
However, after being discontinued by Pontiac, private people or businesses may have them still.
So, NOS, and yes, New Old Stock.
Once discontinued, prices tend to raise, as people still need these parts.
Also, NOS can mean nitous oxide added to the engine for more power.
My 1981 Turbo Trans Am uses both.
George