This might be a silly thought but...

Is there any chance that this is actually a piece of Tarkenton's 1971 Pro Bowl jersey (considering the picture of Fran is indeed from the 71 Pro Bowl) but the card was mis-printed to say "1976" instead of "1971"?

I also went to Beckett's website and did a search and found a picture of the generic non-jersey Tarkenton Pro Bowl card and it also said 1976. Perhaps Donruss simply mis-printed the year on all the cards?