Appears to be a Remington Rolling Block in military configuration. Given the time frame you mentioned, it is most likely chambered for .43 Spanish (11.15x58 mm Rimmed) as it was sold as surplus in large numbers here around that time. Other possibilities are the .42 Russian, .45 Gardner, .45-70, .45-90 and 11.25 Egyptian rounds.
A chamber casting, along with a good micrometer and a copy of "Cartridges of the World" would nail down which.
As with any old military rifle, a thorough exam by a knowledgable 'smith would be mandatory before trying to fire it, no matter what the caliber. Brass, bullets, caseforming and loading dies, and load data can be found for any of the above cartridges assuming that it's safe to fire.
