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Al I vote errors, unless they are proofs; and blank back proofs do exist for most years. I guess the proofs themselves can be variations if they are intentionally changed prior to actual production of the set, such as the 60 Hanley and Cimoli, the 67 Maris Yankee, the 78 Jackson Oriole or the 84 Encased Carlton and Mathews reverse insert pictures
I tend to think of variations as intentional changes, such as the 59 Spahn different date of birth variations , or the trade and option variations from that same year....as opposed to print defects, such as the 57 Bakep or the 58 Herrer, even though they are now viewed as variations