I had a Joslyn conversion carbine in 58 rim fire. Like the Snider and Allin conversions, it was a way to convert a 58 muzzleloader musket to cartridges. It had a side swinging breech cover with a firing pin and cam extractor. Unlike the Snider, in which the breech block swings down into a trough, the Joslyn was more like a cap that swung over onto an exterior flang. A rather weak breech system IMO.
There were some mighty peculiar experimental and limited availability odd guns back in the mid 1800's. Lever action pistols, 12 shot revolvers, LeMats, Colt revolving 12 ga shotguns, Harmonica guns. Rim fire, pin fire, needle fire, centerfire,
I don't think firearms have gone through so much invention and evolution at anytime in history as the early to mid 1800's.