The S serial number prefix was actually first used on K frame revolvers around the end of World War II. I can't remember if before or after.
It was afterwards. The S prefix indicates the presence of the sliding hammer block that is still used today.
FWIW S-prefix K frames and N frames can be distinguished by numerical range; K's are 811,xxx thru 999,999 while N's are 62,xxx-333,xxx.