That's a 56" Turner sling, just the front piece.  I use the same sling on each rifle at Camp Perry, dispensing with the rear portion, which I still have should I need to rig to carry.  Glad the Roosevelt match was on the shorter range last year, cause I had to jog back to my car to get it!  Nothing like getting your heart going early in the morning!

I no longer see the mount being sold by CMP, I'm pretty sure this was the maker, there's some pics of the mounting too!    (You need to Login