The MM were hand punched into the die until the late 80's I believe. This is a good reference, to help you identify errors, http://www.error-ref.com/
Actually yes is an error. Though not worth over 25 cents. Filled "D." Just when some metal flows through, like a die break almost. -SC
Loads of mint marks are filled or barely identifable. I think the mint never checks thoses or low on the quality list
Like a Little Sweetie I knew in highschool. She needed two phone books behind her and almost a queen size blanket that she sat on. I took drivers ed with her.