The reason the center of the mintmark is not formed is that the post that would have formed the center of 'D' has broken off the die
ah, interesting. i'd ask you why they're not considered special but I looked through my little stack of quarters and I've actually got like 3 so i'll assume they're common. I'm pretty sure I have a philly mint filled as well and I've only got like 3 or 4 P mint mark quarters. i'd just never paid attention before and I'm thinking it's more common on quarters cause I don't think I've come across any pennies like that.