The "S" is the old standard for an optic railed gun, hence; SA M-7S. These were post ban models. Now, if the gun is so equipped with an optic rail, it rates an "R." Ex: SA M-7A1R or SA M-7R.
The only other time you'll see an "S" on Arsenal weapons, is when the variant is an underfolder.
The only other time you'll see an "S" on Arsenal weapons, is when the variant is an underfolder.