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AlLemke did an article on them in SCD and they are in the big book now. Variation is not all that exciting. On the back the card can be found with and without a circle around the copyright symbol . On the version with no circle, it can be found both with and without a period after the A in U.S.A.
Tough to run down since most sellers do not know about it, no one shows back scans of commons, and even if you ask them to look they either don't or don't really know what to look for if they do