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Looks like a Remington Split-Breech Carbine. The military large frame carbines were 50 caliber and the small frame ones were 46 caliber. Your carbine would be one of the civilian/sporting models that were made with parts leftover when the military contract was up. They were made in various calibers, it is estimated less then a 1000 were made and I think that number is high. I have only seen two of them in 25 years of collecting rolling blocks.
Here is a photo of both types of the military carbines that I have in my collection.
46 cal split breech carbine - Copy.jpg (33.73 KiB) Viewed 3634 times
I would think that the guns were made from 1866 to the early 1870's.
I think that there would be low collector value for your gun because of it's condition.
The gun is in relic condition, don't know if the rust on the gun is from being left out in the elements or from a fire. It is missing some of the screws on the left side of the frame and the sling bar, these parts would be very hard to fine. The screws could be made by a gunsmith. At one time someone was making reroduction butt stocks and forearms for this large frame carbine, but you would still need to find the correct butt plate and barrell band to hold the forearm to the barrel.
