Derek Grady (SGC's head grader) pointed something out to me at the National a few years ago after rejecting a PSA graded N172 for crossover. Apparently a thin layer of paper had been removed, presumably to get rid of glue or pencil markings. It was a fantastic job and did not have any of the obvious signs of rougher skinning. Under a loupe however, non-skinned N172s have a light shimmer to them on the blank backs when compared with skinned examples. It is something that is hard to explain and has to be seen, but if you have multiple examples of skinned and non-skinned N172s and compare them under a loupe, it will be clear.
JimB
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