1742 Halfpenny - Don't Know What Happened To It

This coin is otherwise uncirculated with original luster and mint color surrounding the rims and letters that is still reflective. However, I have never seen surfaces pock marked in the manner this coin exhibits. The surfaces are very finely porous around the dark areas, and there is a carbon like dark stain, mostly on the obverse, but also wherever the surface impairments are. I don't really believe it is corrosion from the environment, because the entire coin was not affected, it appears to me to be more of a planchet issue prior to strike, or perhaps someone put some kind of corrosive material on it, then removed it. I really have no clue.