I certainly don't expect it to be unusually rare but I am curious as to its provenance and I am hoping y'all can help. It appears to have been cleaned abrasively at some point based on the marks on the metal, almost seems like someone took a wire wheel to it but I could be mistaken. Daryl and I would appreciate any information y'all could provide on this rifle and I can certainly provide more detailed pictures upon request.
I don't think anyone can provide you with "its provenance" beyond the family history you already know.
The rifle is the product of Henry E. Leman of Lancaster, Pennsylvania. Leman opened his gun shop in 1834, and his company produced guns until his death in 1887. He was likely the most prolific producer of muzzleloading guns. The 1860 census reported 62 workmen and an annual production of 5,000 guns. Charles E. Hanson, Jr. estimated his total production numbered "over 100,000 guns, perhaps as many as a quarter of a million."
A large quantity of his rifles were of the style that you pictured. A full stocked rifle with a straight octagon barrel that varied in length from as short as 32" to as long as 42" plus. Large caliber rifles were made for the Western trade and small caliber squirrel rifles for the Eastern trade. His rifles often sported a patch box similar to the one on the rifle you pictured or one of two versions of a cap box.
One of the common cap boxes is often referred to as the "4-screw" cap box and is shown below.
The other is referred to as the 2-screw round "fleur-de-lis" cap box.
Some gun historians have attempted to assign a chronological evolution to these patch and cap boxes with the patch box on the rifle you pictured as the earliest, followed by the 4-screw cap box, then the 2-screw "fleur-de-lis" cap box used on the later rifles.
This doesn't seem to be the case. There are a number of Leman rifles with the "fleur-de-lis" cap box that are dated 1840. There are also a number of percussion rifles like the one you pictured that certainly date after 1840, and possibly much later, with the supposedly early style patch box. It seems more likely that the Leman workman installed whatever patch box or cap box he pulled from the parts bin for the rifle he was working on. All three were likely used through most of the years that the company operated and randomly incorporated on the rifles.
I see the patch box on the rifle you pictured is engraved and the thumb piece on the top of the wrist has some worn "wiggle" engraving. The muzzle cap and entry pipe should also have some "wiggle" engraving as in these pictures unless it was removed by the aggressive cleaning you describe.
You might notice that the Leman rifle I have pictured above has artificial striping on the stock. This was a technique that Leman used often. I can see remnants of the artificial striping on the butt stock of the rifle you pictured.
The "basket weave" pattern checkering on the wrist is common on many Leman rifles as well as other makers of trade rifles in Lancaster and Philadelphia.
The rifle you pictured fits the pattern of a Leman trade rifle. Many of these rifles were sold to the US Gov't for trade and treaty rifles as well as private fur trade companies for the Indian trade. The same pattern was also sold in the civilian market through hardware stores from the East Coast to the Rocky Mountains. Your rifle is one of the latter and could have been purchased in Missouri or brought there by a settler. It could date before the Civil War, but could also be later.