Looking at the overlays, it could be a clash, or it could be some type of die gouge or abrasion. http://maddieclashes.com/MDC-CA1c-1963-01.html
Trapped gas (in planchet) expands after strike, giving coin a bulge that may or not appear on both sides.is this not what happen here?
Then how else do you explain having more than one coin with the mark in the same area, if not on the die itself??