I need your help guys. My cousin has a mil spec 1911 and I used to have a bitone xd 45 that has what looks to be like etchings inside the slide. Looks like initials or a number. Its pretty crude looking, as if done by a handheld drill or something. Now I've had other xds that don't have this. Can anyone explain to me what it is for and maybe why some don't have it?