Ha! Been away for a while, but still watching. I was under the impression that the roman numerals were put there when the rifles were being made so the parts would not get mixed up. So gun "a" would have XXI marks where gun "b" might have XVI marks on all of the parts. I found a trigger plate with III marks on it and a short distance away also found a ram rod holder with the same III marks.
Try to get some pics later.
Dating cast brass serpent side plates... nobody has done it, but there is noticeable variation over time. From what I see, the tail with the screw hole is the easiest to compare with complete guns with dated locks.