As I said before, the service dept. S is indeed the only legitimate candidate for a letter at that location, but if you guys can "see" a double-struck S comparing it to the standard singleton, you've already had more whiskey than I've had this fine afternoon
The S stamp on any parts, however many, indicates the gun did not pass final inspection and was returned for a detour through the service dept. before shipping, for attention to those parts. So it does not indicate any later return. A lot of Victorys have various S's on different parts, a result of the fast expansion of the workforce in 1942 with inexperienced newbies.