SCD recognized the 52 Mantle, Thompson and Robinsons as variations. Not so PSA. I think the reason they do not get much "attention" is that few master collectors would want to need two of each in their set
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I think DP differences, even if not intentional were a result of intentional acts in the printing process and are variations ( my view only). Is the 55 Robinson a DP ? If not, I would view it as a recurring print defect, and agree that if you pick any card from any Topps set and look hard and long you will eventually find some defect. Some are common and recurring, others scarce.
All are interesting to me. And the hobby has been fairly inconsistent as to what gets formally recognized
I also think it virtually impossible to know for sure whether many or even most recurring print defect were intentionally corrected. I think most were not,
I always enjoy these discussions