I have come to believe if one had the inclination to search endlessly one might eventually find a print difference in every card.
There is a discussion of print lines or errant lines in the 1970s thread on here. I have examples of them in quite a few of my sets. Missing ink or too much ink are other defects that occur quite a bit, some are pretty dramatic.
I have been debating whether to keep accumulating recurring print differences that do not seem intentional, or maybe limit acquisitions to those that I think might gain hobby recognition at some point . But the 61 Fairly shows that to be a hap hazard guessing game at best.
Great detective work, Joe