1. The open rear sight does NOT look like a factory installation.

2. Though there is no good pic of the back end of the open rear sight, it seems to me that elevation in this case could have been adjusted by a (currently missing) U-shaped piece (cut with 'steps') fitting under & around the sides of the sight, which would move it up & down by being pushed back & forth. Marble, Redfield & others used such a system on some of their open rear sights - instead of a screw, or a solid step-cut piece fitting in a slot in the sight.