Yep, any of the open top versions shown on this page;
http://www.uberti.com/firearms/army_navy_conversion_and_open_top.php
It has to be one of the open top versions since the Remington New Army conversion does not allow the barrel to come off. And Tuco clearly pushed out the wedges to allow swapping of the cylinder and barrel. As I recall he even looks down it to check the rifling.
And since the movie takes place in some non descript time there's no reason to think that the conversions are not valid for that time. I gather from my reading that the early cartridge 1873's and conversions existed alongside the cap and ball guns for quite a while. Not everyone had the means to buy a new fangled cartridge gun and the fancy and pricey cartridges when they first came out. Cap and ball existed side by side with the cartridge guns for at least a couple of decades for just this reason.