Very nice indeed, I'm enjoying the additional pictures. I'm still of the opinion that it started as a pin-fire, until I can be convinced otherwise. I'm suspicious of the two areas marked in the image below, which would have had metal added to close the pin holes. The joining might not be visible without a 10x or 20x magnifying loupe, and good light. The band of engraving on the barrels might have been added to further disguise the added metal work.

Of course, that would mean the extractor was added to the barrels at the time that firing pin assemblies were drilled, which in turn means the lug on the action bar was added to work the extractor. A big job in my mind, but probably not to a gunmaker back in the day.

Anyway, this is where I would look first:

[Linked Image from i.imgur.com]