It's
Machine Doubling. MANY mintmarks on silver coins of the 40s, 50s, and 60s look like that, which is one reason why I have little interest in silver coin RPMs. WAY too many coins being called RPMs that aren't, and way too many RPMs that are also damaged by
Machine Doubling.
And the correct term is '
Machine Doubling' - not '
Mechanical Doubling'.