The rib might have dovetail grooves on the sides of the rib near the oval hole. Perhaps so a dot sight can be fitted on? The oval hole might be an anchoring hole?

One reason a .500/416 was not more popular is the cartridge has recoil approaching a .450 or 470. So why not just get one of those?

A 450 gr .416 could out do a 480 / 500 gr .450/.470. But the usual bullets is a 400 gr. Of course doubles may not be as versatile as a bolt action with different bullet weights.

Still I have always liked it, but my 450 happened by first. Now I am keen for a .375 s by s as it is better as a medium game rifle as well. .I do wonder if 416 barrels are possible on a custom set of Valmet barrels.