David, I'm also in the process of ever so slowly building a swivel breech using Dave's book as a general road map. Don't get me wrong, if you follow Dave's plans to the letter, you'll end up with a fine rifle that anyone would be proud of. I'm kind of a lazy person and I tend to take the easier way of doing things if I have a choice. As you know, Dave takes a Front action Manton Lock and modifies practically every part to transform it into a Back Action lock. The end result works great and is time tested on many rifles. I however, chose to use the internals of the readily available L&R back action lock, and simply mounted them onto my lock plate. The only downside to this approach is that the mainspring on the L&R lock is VERY long and, IMHO, makes the lock look funny. You can use Dave's idea of a Clevis Block to shorten the lock, however, I was never able to get the block properly secured to my lock plate in order to withstand the pressure exerted by the mainspring. After several attempts I decided to look for an alternative. Forging a custom Mainspring was one option, but after searching for a while I found a much better and simpler solution. L&R also makes a back action Small Pistol Lock. This lock is for a small Darringer size pistol and has a very strong mainspring. I don't have it mounted yet, but it's the perfect size and I think it's going to work great. Using this mainspring has the added advantage of eliminating the Sear Spring as the bottom of the mainspring, with a little shortening and reshaping, serves that purpose. All in all its been a very satisfying and rewarding project. I still have a ways to go, but I'm still having fun and that's what counts. Whatever direction you choose to take your project I wish you the best of luck. Looks like you've got a lot more resources at your disposal than I have and hopefully it won't take you near as long as it's taken me. Good luck, and keep the photos coming!
Capt. David