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Jeff ShermanAnthony's nailed it -- these are printing errors, with the sheet feed for the back printing going in backwards... these aren't considered "variations" nor do they have the cache of, say, the 1952 Page/Sain error wrongbacks... oddityis a good word for them, and so is error, but except for the novelty value, there isn't much of a fire behind the bit of smoke there....
As far as my limited knowledge goes, the 1941 Play Balls really don't have variations, and really the only one I can think of is the presence of absence of a copyright year after the copyright symbol on the reverse: the cards come either with or without the "1941."
Nevertheless, a 66 year odl error card is always going to be something to be enjoyed.. good luck!!