Blank backs
I am not a graded guy but maybe they do not grade proofs or blank backs or wrong backs. There are some player and general collectors who collect such cards but in my experience they do not usually carry big premiums. There are exceptions, like the 67 Maris as a Yankee or the 77 Jackson as an Oriole, but usually because the front differs from the regular issue cards. The 84 Topps Encased or Head in the Box cards are other examples.
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