It's possible they were "replacement" cards from the Fleer vault. After Fleer went bankrupt, the vault was sold and the cards were distributed into the market. They wouldn't have SN's b/c the cards would have been stamped individually to replace a damaged card sent to them by the consumer. There are tons of examples of cards like this from Collector's Edge, Pacific, Press Pass and so on.
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Largest total PC card collections by Team, then Athlete (as of 3/22/24):
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