This is a pretty strange "tang sight". It's made to mount on the tang area of a Ballard rifle, and accepts a modified Lyman receiver sight. It was on a Ballard #4 Perfection in .32-40 I once owned. I think I'll put it back on another of the same model and caliber that I own.
The receiver sight is milled with two receiver slots, and a large thumb knob to secure the sight. The back of the mount is milled with two rails that have positive stops to return the sight to zero when removed and reinstalled.
Craftsmanship is very nice, and the base appears to be a nicely finished casting that is blued. Spacing is the standard Ballard 1.125" holes. The Lyman has 125 minutes of elevation, which should be good for most mid range work.
Wonder who made them, and how many might have been made?