yup, I think we all figured what OP's question was, now those marks may refer to anything, not only killing notch and that was my point.
Plastic holly with a wire stem from a family Care package, on the front sling swivel of an M16A1. Easily removable for inspections and other silliness. I also had an eject-stuck-cases cleaning rod held through the carrying handle and an MG belt link crimped to the front sight base, but that was NOT a uniquely identifying feature at that time and place. Unfortunately, I can't find any photos. Off topic trivia, the length of the cleaning rod was adjusted by selecting the number and lengths of sections (two different types with different number & lengths of sections) and possibly a patch tip so it would eject the case but NOT hit the bolt to trip it off of the hold-open. Eventually all non-chromed barrels were replaced and the stuck cases ceased to be a problem.I would think it is just a way to identify your rifle in a dark trench or cave. Just so you wouldn't grab your buddies rifle by mistake. But what do I know?