1981 Topps Pack in 1979 Topps Wrapper PSA - How do they know?
 

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And how did it happen?

It is my understanding that Topps used packaging left over from the previous year on more than one occasion. For example their use of the '74 wrappers on a decent amount of their '76 baseball. I have seen an awful lot of this from the '79-81 football. On some of their '80 rack packs which hold three packs, you see the '80 rack pack wrapper but some have '79 packs and some have '80 as they are visible from the back. Anyway, I'm no expert on this stuff, but there are a few guys who are, so hopefully someone more educated than I can chime in.

It might have come from a sealed case or maybe the bottom card can be seen through the pack.