Hi Old Bull, it might be a good idea to find out what the mount came from originally. It looks like it might be from a marine sniper rifle. It's certainly not from an air gun. The sideways offset would likely be to see around a top mounted magazine (like on a Bren). You could contact Mike Venturino of Guns magazine (also American Handgunner), Google him. If he doesn't know, I'll be surprised. if it is from an original WWII Marine sniper rifle, it could be pretty valuable.