Back in the late 50's through the 60's' Quite a few smiths re-contoured the rear bridge to Mod 70 shape, with some just flatening the top off level with the rear sight chanel.
I picked up a Winchester manufactured one that had the rear bridge re-contoured the same as the front, except the rear ear on the left side was cut down where there was a very small ear, and a Parker Hale receiver sight was mounted to that ear with the lower leg of the site inserted into the hole used for the ejector button spring tensioner.
So as you can see, there are a lot of different configurations running around out there.
Run a straight edge from the top of the front receiver ring, and see how much diference in level there is, also make sure that both the front and rear are parallel to the straight edge. Then make sure that the mounting holes are in a straight line and parallel to the bore.