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Marc S.I think it is less than a dozen at this point. I do have a double of one of the panels -- surprisingly, it came from the same seller on Ebay in a single lot about five years ago. Just weird to see two carbon copies of the panel from the same guy in the same auction, especially given how hard the actual panels are to find.
The cool thing about the panel I showed above is that it is the second one I have now that features two Hall of Famers on the same panel. It is possible that there is a third panel that has two Hall of Famers (Kaline and Campanella), but I have not confirmed such a panel to exist.
Unfortunately, there is so little information known about these, that I usually just defer to myself in defining population / panels / singles
~ms