Your rear sight sits farther back on the barrel than normal on this type rifle, suggesting it was re-breeched at one time. That was a common practice after years of shooting when the breech, or explosion chamber, got burned out/enlarged and weakened the breech of the gun. I would speculate that your rifle was re-breeched in 1871 per the date on the oblique flat, and at that time the barrel was shortened an inch or more at the breech and a new patent breech with percussion nipple installed and dated.
Shelby Gallien