This is a really neat it bit of vintage scope history.
As the advert above says, The optic below, a Fecker 1/4″ tube “commando” optic, has a Heyer scope square installed. because of the optics of those times being externally adjustable, the elevation and windage adjustments pressed against the curved body of the tube. Some one thought that theoretically this could make the value of each “click” not be as true as it is supposed to be. In practice this isn’t really the case but we learn these things over time and the Scope Square is rare because ,..well , It wasn’t needed.
You an see the wear on the scope square because the optic moves in its mounts during recoil.
Obviously you adjust the recoil “return to battery” spring so the optic doesn’t move so much under recoil that the mounts move off the square.
If you want to know more about the Fecker optic that you probably thought was a Unertl, you can read my past article about them below.