Hey John,
Great thread idea! I'd throw into this category any card of a player that was produced after their playing days were over. 1955 Topps would probably be the exception as those cards are considered official rookie cards even if other cards from their playing days are available.
Re: Pottsville Cards. They were produced by Russ Zacko who was the son of the Sporting Goods Store owner (Joe Zacko) that outfitted the Maroons and was the driving force from the 1960s - 1970s to reinstate Pottsville's NFL Championship. They were produced and sold to help fund the on-going fight against the NFL.
The reason why you don't see more sets of these in the registry is not that they are rare but instead aren't worth much. The grading costs to grade this set are much higher than the value of the set. The set can be picked up for much less than $100. I picked up quite a bit of stuff from Zacko's estate and probably have 10 of these sets including some in their original envelope. Here is a photo of the envelope they came in originally.
Hope this helps.
jeff