The aluminum knob on the side looks to be homemade. It replaces the screw used to lock down the vertical adjustment on the sight.
Im assuming the previous owner wanted to able to dial elevation quickly without a screw driver.
I agree it looks home made. Called a “Gib Lock” Allows for Field adjustments, You would need a little Flathead Screwdriver without the Gib
Williams Factory “Gib Lock”
I have a few of these on Muzzleloader Setups, this is a Williams FP-Hawken TK (Target Knobs) With “Gib Lock”
A “Gib Lock” Takes place of this Little Flathead Screw