Herb: Indeed, your Boswell is a very interesting and beautiful firearm! Thanks for sharing it!
My understanding of the Greener development of the crossbolt goes like this:
The greatest stress on a break open-type action (single shot, double, or over/under) is at the top of the breach end of the barrels. If you think about this, you will understand that, when this kind of action is stressed, the widest gap between the barrels and the frame is where the action opens at the top of the barrel breaches and the top of the receiver frame. (The smallest gap is the area between the barrels and the frame that is nearest the pivot pin.)
Thinking about this, it should be clear that the strongest place at which a break open action can be locked is the area closest to the top of the barrel breaches and the top of the receiver frame. This is why Greener located a locking lug at this position in the action.
However, there's a downside to the Greener locking system: In British pass style shooting, the hunter normally hunts with several guns and an assistant, working from a blind. For this kind of shooting, the assistant is reloading while the hunter shoots. For this type of shooting, a flat breach is preferred, since the loading process (and especially the unloading process, if the gun has extractors) is easier if there is a flat breach that is unobstructed by a Greener-type lug. So, the weaker traditional underlug system is often preferred for this sort of shooting. (This is also why SxS guns are preferred over O/U guns: O/Us require a more obtuse opening angle, and are less handy to manipulate in a blind.)
There are a number of arrangements for a moving locking lugs. Some use straight sided lug, while others use a tapered lug that will take up normal wear on the bolt and the locking lug. LC Smith, rather than using a sliding lug, used a rotary bolt that looked somewhat like a comma. The "tail" of the comma was tapered and would rotate into the lug on the barrels.
Another arrangement that didn't lock quite so tightly but required no moving parts was the "doll's head." The lug on the end of the barrel, when viewed from the top of the gun, looked like the letter "T", where the stem of the T attached to the barrel breach. The lug fit into a matching T-shaped slot in the receiver. The lug and slot were tapered on the end so that the barrel would rotate smoothly into battery with clearance, yet lock tightly when completely closed.