Hello again. Not sure what happened here (D/P, D/rotated D). Maybe one of you have seen such a mintmark issue...
It's a plating blister. If you look you'll see them elsewhere on the coin too. Very common on the earlier, copper-plated zinc Cents.
Yeah, there's probably some zinc rot going on. Take a look at these examples of zinc rot- http://imgs.inkfrog.com/pix/coop49/Rise_Fall_Fell_Off.jpg