Ok, you first have to remove the elevation screw & spring to lift up the inner piece of the rear sight. UNDER that is a set screw in the base that's screwed down "locking" the base to the receiver. Loosen that screw, then gently tap out the entire sight.
DON'T just try knocking out the rear sight assy.
Those are supposed to be Roman numerals therefore I's not 1's. Yes, I know what you mean just like when someone writes "new model Vaquero" incorrectly.