Part of the post that forms the inside of the mint mark has broken away from the die. Now the center is not rounded any longer and eventually breaks off and leaves the cent completely un formed. Some call this a clogged die, but the opposite is true. There is nothing there that forms the center of the mint mark.
Thanks Coop and CM for your explanations. I think I understand now: the die feature that presses on the copper and forms the inside of the D would be relatively small and vulnerable to breakage as the die aged. When it partially broke the copper filled that spot on subsequent coins, resulting in the "filled D"... neat!
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