"Rocket can we agree that all things being equal the bearing surface furthest from the hinge is the optimum place for a locking bolt?"
It is the place with the lowest force but that may not necessarily be the optimum place. A rib extension locking mechanism means that more of the action and barrels must carry the force of firing. That means that the action and barels must be stiffer to control flexing. "Stiffer" usually means bigger, which usually means heavier. I don't see any of the locking mechanisms as "optimum." They each have a set of pros and cons for the designer to consider and work with. If one was optimum, that is the one every designer would use. In terms of use, I guess that the high volume of USA made guns with various rib extension locking about balances out the more wide spread use of the Purdey double under bolt system.
Greener, as I recall, demonstrated that a gun snuggly on-face could be held shut with finger pressure during firing. That means that the locking mechanism doesn't carry a lot of load until the gun becomes loose.
