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fkwSince it doesnt have a "W" number, I think they did that because 1927 E210 York is the main set for that group of cards. Just like the 1916 M101-5/4 Sporting News is the main set for that group, and 1917 E135 Collins-McCarthy and 1921 E121 American Caramel are the main sets for those 2 groups, etc.
I just picked up a blank back E135 and think if I sent it in it would came back as a E135 with a blank back.