Considering that everything else is finished, the lack of a hole for a lock bolt makes me wonder whether the lock was held in place by a wood screw through the lock itself. I have owned two original rifles that were made that way and still own one of them. I own a couple of guns that were found in walls, one a 10 gauge percussion double shotgun by J. Donn & Bro. Canton, Illinois, that was found in the wall of a house in Dubuque, Iowa.
Many years ago (about 50) i knew an old gentleman who had been an antique dealer before WWII, back when everything was worth nearly nothing. He used to clean out attics in exchange for the contents. He told me he once cleaned out a very full, floored attic in a large Victorian house near here. When he was all done, or thought he was, the lady told him that there were three old guns down inside an upstairs wall "get them out of there". He laid on the attic floor and reached down inside the wall and was able to pull up two percussion long rifles, The third gun was a percussion double shotgun. It was shorter of course, but he was able to get two fingers worked into the muzzles and pulled it up. When he got it out of the wall he realized that it was loaded, capped and cocked.