As said above, the gun is a first Model American, aka Old Model No. 3. Caliber is probably .44 S&W American centerfire. If the chambers are bored straight through, it's .44 American caliber, if you see a ridge about 1/2 way down, it's .44 S&W Russian caliber, however the gun is pretty early to be in .44 Russian caliber. The hammer isn't cocked, so I can't tell if it's a rimfire, which would make it a very rare gun. If it's rimfire, then the caliber is .44 Henry. If the gun is original the assembly codes stamped on the barrel, cylinder and frame will match. They are usually 2 or 3 digits or letters. Look on the rear of the cylinder, rear of the barrel below the breech and under the left grip. The only place the serial number appears on this model is on the bottom of the butt and on the back side of the right grip panel. The gun appears to show honest wear and use but not too much abuse, so listen carefully, should it decide to talk! It was probably shipped to M.W.Robinson in the early 1870s, but only a factory letter will tell for sure. Ed