I just read the Potter article, and I believe you took his comment about being inverted the wrong way. He was talking specifically about that incident with the dropped letter "A" and the way it landed was inverted. No one can guarantee which way the debris is going to land when it falls out of a die. Chris Score one for Hobo!
I think it a dropped D from GOD. It is too narrow to be a dropped letter from the mintmark and none of the other D's are small enough.
It's an interesting error, but I don't think it would be worth spending $25-50 to have graded. Just put it in a flip or an AirTite and leave it at that. Chris