Geno,
Cool muzzleloader! Any idea what the distance is between the graduations on the rib?
I agree with Lee that the equidistant spacing on the rib markings rules out a sighting sytem that "automatically" compensates for the drop in the bullet. My thought is that the rib graduations simply offer the hunter a repeatable place to mount his front sight. Through trial and error, the hunter would know with load X and distance Y that his front sight needed to be set at Z.
I think the rib would also make a handy measuring tool - kind of like the ruler/scale built into the side of my fishing tackle box :-)
I looked long and hard at the shield engraving. IMO it is a "high seat" of sorts. The ladder and what looks to be a smoking gun barrel sticking out of the "leaves" is what convinced me. I think if it were flames, the engraver would have depicted them a little differently.
It is a fun gun to ponder on a Monday morning.
Ken