What you probably have, BS, is a "Allin Conversion" of a Civil War muzzleloading .58 cal 1863 rifled musket to a "Trap-Door" .45-70, done in 1869.
The Army knew they needed a breechloading rifle, especially when the Indian Wars in the West "resumed," and some of our Soldiers had already been beat up pretty badly armed with the issue muzzleloders but after the Civil War, funds were tight, and we had literally MILLIONS of the muzzleloaders left over...
So a guy by the name of Edwin Allin, who worked for Springfield armory, came up with the "Trapdoor" idea, in 1868, to convert existing Civil War muskets to breechloaders, and it got patented, and Springfield converted about 30,000 rifles from the 1863 Muskets, by rebarreling them and fitting the Trapdoor, hence the name "Allin Conversion."
After converting the 30,000 old muskets, the Army liked the rifles so much they ordered new rifles built from scratch using the trapdoor, so that led to the famous 1873 "Trapdoor" Springfield...they stopped calling it the 'Allin," although his trapdoor was virtually unchanged.
The only thing about yours that doesn't fit is the "1869," since it should be a "Model 1868," I'd have to say that is probably not the MODEL as the year it was actually converted.
And yes it PROBABLY is a .45-70, but I would have the chamber cast before you attempt to shoot it. First, the actual .45-70 cartridge was not standardized until AFTER these conversions were started, so while yours is a later one, and is PROBABLY a .45-70, if I remember right some early ones may be in like a .45-60 or somesuch...
Second, AFTER it's service, someone could have bored it out to say .45-90 or some Buffalo cartridge popular later...
So either you or a 'smith should make a cast of the chamber to be sure...
And even if it IS a a .45-70. be careful firing full house modern loads in it, remember the steels in it are made for BLACK POWDER pressures....now I KNOW that factory .45-70 ammo is supposedly loaded down to avoid problems if somebody shoots it in an old Trapdoor or rolling block, I would be afraid since yours is a CONVERSION to a Trapdoor, not a "Real" Trapdoor.
But having said that, since there were a LOT less conversions made than later from scratch Trapdoors, I'd probably not be shooting it at all, it's probably a bit more valuable than an 1873, and they certainly don't give THOSE away....
If you want more info, try googling "Allin Conversion" or 'Allin Trapdoor" and see what you come up with...I got my info from a couple of books I have....