That looks like a 2 inch Unertl, and the recoil spring is still in operation. I suppose that the Ma Duce would require that as it probably had a lot of recoil.
I believe the rifle that Hathcock used was a M70 Winchester in 30-06 using M72 ammo - probably NM grade. Must have been a chore keeping those Unertl mounts operational in the Nam climate.
I had a friend who served in Nam with a USMC radio operatioin and repair outfit. Said that a Marine came in with an M16icon and asked if he could use the sonic cleaner. The cleaner removed all of the finish, and my friend said that you could watch the rust form.