Any Ideas On This Early Trapdoor(?) Sling?

[J.Q.M.D.]

Greetings all,

I finally got around to listing some of the slings on eBay that I got in that huge lot of Indian Wars leather. There were a few of these in the mix that I haven't been able to identify. They are the same length as the standard 1887 sling. Made from two pieces with a SINGLE HOOK (like the 1870 slings) on one end and a BUTTON (like the 1887 slings) on the other. Does anyone know the official designation of this sling? The only markings are "US" stamped on both sides of the splice. Any information would be appreciated!