Cannoneer, that is very nice Signal Cannon you have . Have you been able to date yours?
Road King,
Those photos are from an auction, and I didn't even bother bidding, because I knew the price would reach a height I wasn't willing to climb.
As intoodeep already said, your gun has to be an early model. The placement of the ejector, the way the striker is attached, and the differences between the breeches (the later model has a shelf cast integral with the barrel, that the breech abuts when closed) suggest it's an early version.
I agree it's obvious that the working breech was thicker than the plate that's now on the gun, but the thickness (along with how far the firing pin mechanism extends rearward) is going to have to be gauged by where the hammer strikes. Even with that raised cylinder that's on the breech now, the hammer seems to be hitting high (at least judging by the marks on it).
There are two holes, one on each side at the rear of the barrel, and their locations seem to correspond with he screws that attach the breech to the barrel; are those holes threaded, or were they meant for pins?