Like others have mentioned, the Roman Numerals are probably assembly markings. This was utilized by Southern Gunsmiths during the Civil War, as most here probably know. They'd mark the barrel, barrel channel, lock, etc... for re-assembly purposes. I imagine the practice has been around a long time.
The question I'd have for this particular barrel, especially if it was found in the south and is potentially shortened, is if the Roman Numerals were added while it was being altered to percussion. I doubt it would be for the purpose of the Civil War, but perhaps it was just a simple Southern Gunsmith giving himself some reference points with Roman Numerals while working on multiple projects.