Pull the lock and barrel out and see if the undereside and backside matches finish and wear. See if the stock underneath could be that old. You have to use your detective powers on these. How is the nipple area. That area should show a little pitting I would imagine. From here is looks good.
Looks original, as far as can be told from the photos, although not in the best of shape. The cleanout screw is NOT original to a Model 1863, though, and appears to be somebody's idea of a later "improvement" to aid in cleaning.