@dtp, first welcome to CCF. Second, it looks like the copper plating Near the letter S may have a couple small holes in it. That allows the zinc core to begin to deteriorate and sometimes causes oddly shaped bulges or bumps. Your pic isn't clear enough for me to say whether the same is true for the mintmark, but it seems likely.
"The danger we all now face is distinguishing between what is authentic and what is performed."