Except in some isolated occurrences the rib matting Will terminate before the end of the rib in one way or another.

my one observation looking at the close up photo's of the muzzle, even though the OP said he cleaned up the muzzle to see the keels, when I look at the muzzles of most of my guns, the edges of the barrels seem more rounded, both from shot rounding off the inner edge, and the muzzles being "polished" by the ends of gunslips, resting on the top of the foot etc.

How long are the chokes ?