Berhard got it right. Nice work. It's a grenade Launcher sight for the WWII M1 rifle, M1 Carbine, and the 1903, 1903A1, 1903A3 rifle Abusive comment hidden. (Show it anyway.)
It's a grenade Launcher sight for the WWII M1 rifle, M1 Carbine, and the 1903, 1903A1, 1903A3 rifle