I found an AR15 today with a mystery upper reciever. The lower is bushmaster, which I realize doesn't really mean anything in upper identification. Anyway, here's a description. Its a flat top with a folding front sight which I sort of think may be removeable. 16" barrel, not a thin barrel, or M4 style barrel. No markings like caliber, twist rate, nothing like that. I called them tonight to see if anyone there might know what it is, and the guy said the only marking he was able to find was something that looked like an old fashioned keyhole. Any idea of who made that upper? Or why it'd not have any kind of manufacturer's marking? Thanks.