I really doubt GM is still making, or even stocks body parts for a 1967 model year truck.. so these 'original GM' parts are actually 'officially licenced' parts that 'meet or exceed original specs'.
The difference between garden variety repop parts and 'official GM reproduction parts' in most cases is just a sticker.
GM gets a kickback on each piece sold.. which is why they cost more. The seller passes that price on to the customer, and bumps the price up a hair higher than that to make it worth their while.
