For anyone who read my earlier posts regarding the 20 ga Davidson Firearms S x S, I'm happy to report that I finally fixed the darned thing!
Whoever worked on it before got the cocking levers and hammers mixed up (they're hand made and fitted for each side of the box lock) and put punch marks on them to pair them up. Once I figured that out, things became a little simpler.
I had to make a mainspring tool to reinstall, since I couldn't find one commercially that works for an Anson and Deeley box lock.
The metal was worn somewhat on the forearm piece that presses against the right side cocking lever, and after getting everything back together and discovering that I really had to crank down on the barrel to get the right side to cock without it slipping past its hammer tang (which was the original problem), I discovered that my next part of the project is to build that up with a little welding.
The cross bolt lever spring's tip was broken off, and the slot it rides in is worn out also, but it is such a pain to get out that I may fashion some kind of coil spring to return it, even though it will remove it from the ranks of "restored to original."
Nonetheless, it now shoots both barrels!