I do believe MM's are sometimes somehow "erased" before others are added, though not always and I don't know how.
Here is a list of 1961-D photos from Coppercoins.com (love that site)

Note, the fist picture: The MM is rotated 90 degrees and is thin, look at the corners...LLC is sharp, LRC is round...look at my pictures; they are not inconsistent, mine is slightly lower which given they were done by hand is not unusual (to me).
Why the sharp post sticking out midway below the left edge of the mm if it isn't consistent with picture #1?
Look at picture #4...consistent in 61-D's is the thin under mm, which appears an attempt was made to abrade/erase the mm before the corrected orientation/style was punched.
I don't have the software to measure it but maybe someone out there can?
I am not trying to be right here, It could be a "wall" pushed up from a very hard hammer/punch strike or something else mint people would know but I don't.
Thanks!