The semi-circular detail that creates an outline on this object, can certainly begin to resemble a partial S or U, if one looks long enough with one eye closed (rather like looking at clouds and seeing all sorts of things

).
I believe though, what we are seeing in that semi-circle though, is just an illusion created by a crumbling material along with a raised area. Over the years, I've seen and dug quite a few Rev War buttons, including several Continental USA examples. Usually Colonial era pewter buttons begin to crumble on the edge first, and the center area will be nearly as smooth as the day they were made. The item in question here, still retains a nearly complete circular shape, and yet the surface it bubbly like and somewhat flaky. This is completely opposite of what we would normally seen in most recovered Rev War buttons.
Here is one of my best Continental USA buttons, and an unusually rare variety at that.
CC Hunter