Ted,
Shotgun design till 1875 was based on lever cocking inventions, culminating in the Murcott Mousetrap in 1871.
The hammer gun is outwardly similar to the sidelock, yes, but internally they differ. The sidelock lockwork departs from the simple tumbler with a safety bent of the hammer gun enough for the differnces to be called radical, in my opinion.
The step from hammer gun to hammerless sidelock is described as "evolution", a word alluding to a gentle process. Considering that it was due to the advent of barrel cocking, a new forend (again the workd of Anson) which made it possible, the word evolution does not really fit. It was more like a revolution which bypassed the problems inherent in lever cocking actions, especially ejector actuation.
Personally I love lever cockers, the Darne, the Ideal, even the low end Tarzan, are firm favorites. There is something fascinating about packaging all the action in one self contained assembly actuated by its own lever rather than mixing in the barrels to do the work. However, the market voted for barrel cockers.
Re the stocking issues of Round Actions. This is an issue with the Dickson design, not all trigger plate actions. There are plenty of trigger plate lock shotguns out there, almost all Over Unders have that system now, and the stocking issue has been bypassed by the stock bolt. naturally the stock bolt would not go on a best gun, even though Greener had positive things to say about it.
