That's not really a letter 'S' (but it sure does look like it on the pic). That is just a round raised part that is on the rivets I have seen. The green patina just makes it look like a letter. I have found fired bullets and the belt in my sig a few miles away, but I think those were random lucky finds.
No idea. Does brass get a green patina to it ? It kind of looks like a rusty copper color where there is no green on it. No letters or numbers on it anywhere I can see.