The 2011 Standard Catalog lists the 1931 W-UNC Strip cards and the 1931 W502 Strip cards separately. At the bottom of their description for the 1931 W-UNC set, they say, "This issue's checklist is identical to the 1931 W502, the only difference being these cards have blank backs."
Because they are cataloged (and graded professionally) as different sets, I have always considered them different cards. I can tell them apart only based on the contents of the back of the card.
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