I got a 15-round-linked belt of .50 cal “used” ammo because I wanted different headstamps. After tearing the belt apart and looking at the rounds, I decided to look at the links (expecting identical stamps). I was plesently surpised by almost not having same stamps. Many of them have “S#”, like “S2”, “S17”, S8" etc. What is this “S” and why such a variety of numbers?