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Marc S.there are enough sets out there that are singular in nature, that I do not think it is hard to suggest that Herpolsheimer's is as well. Alpha Engraving, Allegheny, etc.
And since it is the 1921's only that are considered unique -- I think the fact that they were all found in the same place, etc. lends credence to the belief that they are unique. Perhaps a test run that ended up being vetoed, and the cards ultimately ended up with a dealer...
Anyway, there are better experts here than I on this cool issue.