Without a chamber cast all we can make is a guess. Rolling Blocks were also chambered for the 11mm Egyptian which has a larger base diameter and rim than the 11mm Spanish. They are usually stamped on the top of the barrel with various Arabic markings, but not always: those diverted to France during the Franco-Prussian War, 1870, are not. Also, at one time some importers reamed out the chambers of some 11mm Spanish rifles so they could shoot 11mm Mannlicher ammo. Some were stamped "11 MANN" and some were not. At the very least take a caliper and try an get an accurate measurement of the chamber where the back end of the cartridge would be.
Your description of the rear of the barrel would indicate an Argentine contract that was originally chambered for the 11mm Spanish. If a 11mm Spanish case is loose in the chamber I would guess it had an 11mm Mannilcher reamer put into it.
Rolling Block...not sure what I have
Re: Rolling Block...not sure what I have
The breech end of the barrel indicates it is probably the "Argentine Model". On the top flat is stamped "Modelo Argentino. 1879 E.N.". Some rifles had this ground off and others (contract over run ?) may have been destined for other governments and, therefore, left blank. The patent dates on the tangs serve to indicate modifications. There were 10's of thousands of Rolling Blocks with the ending date of March 18, 1874 made for various governments or groups. Check out www.militaryrifles.com under the Remington or Rolling Block section and Argentina. George Layman's soon-to-be-published book on Military Rolling Block Rifles will touch on countries that used the Argentine Model.