Looks like a way to put a strap attachment point on a barrel. Thicker part tightens and pinches the barrel, the set screw adds pressure to keep it in place and the hole is to either allow the barrel clamp part to flex and/or to allow the front sight post to pass through?
I don't know for sure but based on the shape that is what I guess it was designed for.
Bottle opener, tooth pic, the loop is for a spool of floss
Bottle opener, tooth pic, the loop is for a spool of floss