Nathan, quality is a very sticky word, but the inletting and engraving are very similar to a Joseph Manton that I picked up. Sounds like the same proof marks as well. My understanding is that the platinum blow plugs went out of fashion in the 1840's so I would suspect it of being slightly earlier (though going out of fashion in the 1840's would hardly preclude its being made in that decade). The stock appears to have virtually equal drop at comb and heel, as does mine.
"Track of the Wolf" has a number of percussion muzzle loader SxS guns for sale (which do sell) as a point of comparison for value. Condition of the barrels will be of importance to its value. I would not sell it for $400 (even to me).
I hope it proves to be a good shooter for you.
