Replica Arms .45 caliber Spanish made barrel. There were or should I say used to be a lot of companies that made rifles. They purchased parts from whomever they could find them through. Some were real good some were just a put together rifle. The Spanish barrel you have was common. The max powder charge is 100 grains loose powder for them NORMALLY. In the .45 caliber they normally take a .440 ball and patch. 40 grains of 2f powder would be a good start once your sure the rifle is well cared for and safe to shoot.
The breech plug in the back end is welded normally. So leave it alone. The drum on the side also does not come off. And many times there is a clean out screw in the drum (area the nipple screws into). Over time they become almost welded in there from use. Leave them alone. If you try to take it out, normally all you accomplish is messing up the screw head.
I would clean the barrel real well. Put a bore light down it and inspect it close. If you have any doubts of the rifles integrity, take it to a gun smith and have them check it. Again, loose powder only, and small charges are the way to go. Your accuracy would seldom come at 100 grains of powder. Your max load might even be less then that.