One trick that works well is to tin the rear right with solider and coat a screw too, Run the screw into the hole and let it cool. Next cut the screw off flush and polish it down to the surface of the rear ring. Now, tin the bottom of the base. Screw it down to the front and put a clamp on the rear to apply a bit of pressure. Heat until the solder melts so the base sweats to the receiver.

When cool take a #27 drill and spot the steel under the base so you can then drill with a #31 drill and it will be center of the hole in the base. Tap the new hole. Sock down a screw and you are ready to clean up the solider and blue the receiver. This is a permanent installation so you want all the other polish and metal work done first, but when done this way it's an excellent way to secure the base. Don't worry about the temper of the steel. The top of the rear ring on a Mauser holds no pressure and could be made from the softest steel and still work fine. And if you use solf solder it never get very hot anyway.

Hope this helps you.