Well has anyone looked at the omm possibility? As there is an omm for the 52 D http://varietyvista.com/
Copper, being a soft metal, can be moved around on a coin when struck by another object. The MM on your coin has numerous hits and I believe one of more of those hits moved the metal to what you are now seeing.
And no either did I...however if you notice the top of the D there are two serifs . As well a serif on the bottom as marked...and again not saying it or isn't ....as you are correct its a 70 year old coin well circulated cent... that has a lot of dirt in the D loop...but unfortunately I am out of acetone..so until I get some... it will sit awaiting a bath. Just wanted to hear others opinions as yes I do have a Lincoln album...1909- 2007 however they are not my strongest interest as other series are.
Now in 1952 one of thexe 3 mint mark styles were used to product the 1952 D Lincoln cent. Mms 003 at the bottom photo Mms 004 middle photo Mms 005 top image. The omm D/S