I doubt that the scope mount on your rifle is a custom job.
The Chinese have made about a dozen different receiver cover scope mounts for AK rifles under the BEC,Norinco,Red Star,ChinaSports,and Leapers brand names. Here is a pic of two of them.
The scope ring rails of which I have seen 4 types of were attached to the receiver cover by welding,spot welding,rivets,or screws. As you can see with the Norinco mount there is a piece at the front that goes under the rear sight base. The two screws are tightened to make the cover tight on rifle. Mounts were imported that had only one screw in place of the two screws on the front. Also mounts that didn't have any screws but still had the piece that went under the rear sight base.
The Leapers uses two screws on the back to make the cover tight on the rifle.