This is our latest find- Vietnam era rifle carrying straps for carrying a rifle on the side of an Alice pack. The strap has a pocket that goes around the butt of the rifle, a wire hanger that attaches to the pack, and a longer strap that goes up, passes through the upper strap on the pack, and then hooks over the barrel with a sewn in loop. The strap is pulled tight and the rifle is securely strapped on.
Made or use with the M-14, it can also with with AR-15 rifles or pretty much any long gun that the strap can hook over the barrel.
Your Alice pack needs to be one with the two metal eyelets on the lower webbing on the side. If your pack does not have the eyelets (later ones deleted them) or you have a newer pack with MOLLE style webbing you can still use this, but you will need to add a MOLLE/Alice adapter that has the eyelets on it.
Unused, still in the factory bundles from when they were made in 1966.