I've found several different thread pitches used on these detachable side mounts and if memory serves even with Weaver. If I understand you original post the thumb screw threads were damaged. Those threads are a special fine pitch size. Recently had similar problem with same mounts and rifle series. I lucked out being a collector by taking mount off another rifle before selling. Believe the taped holes in receiver are not the current size but something like 10-32 machine screw. Apparently there were several makers who sold that side mount style besides Weaver. You may have to do some research in old gun magazines, catalogues etc. to determine what make you have.