Looks like where a rear sight might have once been. Are there tapped holes near the end of the muzzle too? If so it could've had open sights at one time, though that would be sort of odd on a heavy barrel. I don't see why someone would've done that.
It could also have been drilled for a mount for a scout scope. Once again, I don't see why someone would've done that when there's a perfectly good set of holes drilled in the top of the reciever that use the same two-piece base as a Remington 700. Some people prefer a scout setup, I'm not one of them. Especially on a heavy-barreled rifle.
I'm going to go out on a limb and say it's not factory