Update:

I just my Rigby back from JJ today and I am very happy. He built a red dot mount using the thumbscrew that is torqued down using a cartridge case as 470Evans showed above. However, to keep from cutting into my rifle, he built a new rear sight piece which provides the mounting surface for the red dot mount. It contains a rear sight that can be used as normal if the red dot is removed. Also, the rifle can be put back to 100% factory by simply drifting off this rear sight piece and replacing it with the factory one standing and three folding leaf rear sight. Quite ingenious really. Please see pics: