A counter mark. Adds interest to an otherwise so-so coin.
Unfortunately it is highly unlikely that you will find out when, where or why it was applied.
Jamais l'or n'a perdu la plus petite occasion de se montrer stupide. -Balzac
Now, I already hear your objections: The last letter cannot be an S but rather a C, G, O or (much less probably) a Q.
However, it would be odd that the punch was able to reach so far low at a point which is the lowest on the coin and beside one of the high points, the back of the head. How could the punch get an impression in there and not at least as low and deep on the back of the head?
I suggest therefore that the lower curved part is just a mark on the coin and that the top is that of an S.
Unfortunately not. There is a W. GIBBS in Brunk countermarked on an 1816 large cent (which info is found in his book though not on the web page), but "W." is such a common initial that it's likely that one is British and the other American. There is no picture, so we can't compare the punches.
It's unlikely we'll be able to identify him. But if I'm right about the name, then one should hope that the same countermark might show up at some point on another coin. And having a full name makes it much easier to do a search.