I milled my 9.3x74R Merkel’s rib with a small 60 degree groove on either side of the rib, and then milled a small shallow locating slot on the top of the rib. The operation is not terribly hard, nor is it terribly intrusive. The end mill bit is a small 60 degree bit that looks like a triangular or pyramid shape with the tool shaft on the top of the pyramid.... The grooves are cut to just a couple of thou deep with the top of the groove about 1 thou below the top of the rib. You don’t need much for the base of an RMR to affix solidly to the rib. The rectangular locating hole is machined into the top of the rib near the front end of the grooves and is only about 2 thou deep and is oriented at 90 degrees to the rib.

Trijicon makes a mount for 11-12mm ribs which fits perfectly on the modern Heym top ribs - the Heym factory has made standard this machining work to accommodate the RMR.

For the Merkel, Trijicon has a mount for 14mm ribs.

On my Merkel, I (very carefully) did the machining myself using a small hobby mill. I’m NOT a trained machinist, so for a professional gunsmith, this is not difficult or horribly intrusive surgery. In any event, I won’t own a rifle that I can’t shoot, and if an RMR makes life easier for my ageing eyes, then so be it. A rifle that I can’t shoot has no value to me whatsoever so I’m not terribly fussed about cutting into a good double to make it usable. If it’s a museum piece, then it would be too valuable to shoot anyway so maybe that’s an indicator of what mods to make or not. The OP had made mention that his rifle was rechambered at some point so to my mind, that would kill museum piece value anyway so have at it and make it useable!!