You have a Spanish contract rifle made by Remington. If the breech block has a concave portion where it rolls under the barrel and the last patent date is 1866, it is the 1st contract. If it has a flat breech block in the same location and the last patent date in 1866, it is the 2nd contract. If it has a flat breech block and the last patent date is 1874, it is the 3rd contract. The marks on the sides of the barrel are symbols of the Spanish royal family. If you look closely at the top of the barrel in front of the receiver you should see either an "R" or portions of such. Many rifles were poorly stamped. This was applied when the rifle was converted to the Reformado cartridge. I don't have the measurements for the Reformado chamber in front of me but I have several rifles and carbines chambered for this cartridge. None of them are interchangeable. This leads me to believe that conversions did not all originate in an arsenal but maybe some field facilitys.
I generally use the .446" Mauser bullet for shooting Reformado-chambered firearms.