I have read numerous times that card sharps from days gone by became proficient at noticing the various signature discrepancies of the texture and inking on the

I have read numerous times that card sharps from days gone by became proficient at noticing the various signature discrepancies of the texture and inking on the back of playing cards so they would not need to crimp, clip or ink a card to mark at least some of them in a particular deck in play. Basically the more proficient you were at spotting these details, the more "marked" a deck might as well be for you.