Since the caps aren't dated (as far as I know), there's no sure way to determine whether they are original to the car, or were replaced at some point. In 1969, and certainly into the early '70's, most gas stations pumped the gas for you (I worked for both Mobil and Union 76 while in school, pumped many a gallon), so leaving the cap tucked between the filler/plate door and letting people drive off was a rare occurence. How many caps do you see on the side of the roads nowadays ? Plenty. How many caps failed emissions testing and had to be replaced ? None - we didn't test then. Point being - I don't think I have ever replaced an early gas cap, dating back to my Corvair and Bel Air days. If most caps are originals, and survivors have the same spread of caps, it's going to be a difficult question to answer.
Just my two $.01's - but keep the discussion going. Somebody has to have a clue. Survivor owners ?