Weren't all period percussion caps corrosive? Looks like the barrel was cut back to get rid of the eaten up breech. The gun shows consideable useage (or abuse). It appears that it was made as a nice tight little upper susq region gun and then modified considerably during the period of use; things like additional inlays chunked into the wood. nails used for barrel pins, missing inlays and so on, aside from the huge cut back on the barrel length.
One other possibility for the big trim is that it could have been altered for the Western Trail, or as Kindig theorized, an aging owner had it shortened to make it easier to use.
Some random thoughts.
Dick