Well, just to drag this out a little longer, as long as I had them out of the safe I decided to break them down, run a patch thru the bores and lube the hinges. I noticed another variation, this time in the forearm loops.

Early guns have a lovely reinforced loop with a single hook --



Mid period guns still have the reinforcement but an added spur which seems to force the forearm back against the hinge.



Finally, guns above about 133,xxx have lost the reinforcing feature --