William Dodge and his brother were inventors in Washington DC who in the 1870s patented a bunch of different improvements to the Remington Rolling Block, among other guns. This particular one I cannot identify with a specific patent, but to my eye it is a way to give the Rolling Block system a safe way to be carried with a loaded chamber. Using Dodge's patent system here, the hammer can be lowered on a loaded chamber in a way that securely locks the hammer from falling and locks the breech from opening. This ensures that the action isn't accidentally opened while carrying the gun. To fire, one need only cock the hammer back slightly and it is ready to use.

Despite there being no Dodge marking son this example, I am pretty confident that it is form the Dodge brothers because another example of the same system is in the Smithsonian collection attributed to Dodge - and Dodge sold his own gun collection to that museum in the early 20th century. It would stand to reason that they would have properly identified it.

That said, this system was never put into production. The US Army did experiment with a similar sort of half-cock safety on the Rolling Block carbines, but not of this design.

Update: It has been pointed out to me that the specific patent for this system is US Patent 113408A. It was specifically intended to keep the breechblock from opening in case of a cartridge discharging when the breech was closed (such as in case of the firing pin being jammed forward by something).