When i was taking pictures of all my coins i noticed that this Jefferson Nickel had mintmark D Only thing is that the D is not pressed well i think. The inside of the D is not impressed, but equals the hight of the D's rim (hope you guys understand my explenation ) I know the pictures are not verry clear, but its hard to make pictures of coins
The inside of the D chipped off the die. This happened a lot back when MMs were hand punched. It's more of a die state than an error.
Filled devices were common on the '40s and 50's coins as the result of chipped/damaged dies. If you think of the "D" in reverse on the die, the center of the "D" would be post-like and easily broken off giving you a filled center. The same goes for the centers of the letters like the "B" in Liberty on the cents which is commonly found filled.