I believe it is just damage as the size and shape of a D would not be such. Looking at the other letters near by, U.ONE and the DIM on the other side, the damage made have been done in an old coin dial public phone or amusement machine or just was hit on that side of the perimeter. jim
It may be that the MM was over-zealously stamped into the die, with made it dimple a bit. With wear, it looks like a line.