I checked "Kentucky Gunmakers 1775-1900, Vol.2 Biographies" on Mosias Maupin, and then Vic Paul's book on Missouri gunsmiths. He arrived in MO several years later than Boone, arriving in 1803 or 1804, and is first documented in MO at St. Louis in 1804. It was the 1803 Louisiana Purchase of that area by our government that enticed him to make the move from Kentucky.
Vic Paul states there are several known guns marked "M M" stamped or engraved in block letters on the barrel, but the actual maker is unknown. Since most of them are later percussion rifles, made well after Mosias' death, the prevailing thought among Missouri collectors is that they were probably made by his son Daniel, also a gunsmith, who may have kept the M M marking of his father to help sell the later rifles Daniel made. There was no reference, or statement, in Vic's book about any Mosias Maupin rifle actually being known, or even of one of the "M M" marked rifles actually being documented to a Maupin family gunsmith. Rather, it is the most logical and compelling assumption by knowledgeable Missouri collectors.
From my research on the KY end of Mosias' career, and the early date an actual Mosias rifle would have to be, I'd think the barrel signature would probably be in engraved "M M" in script and the gun would have the earlier lines of a lighter, more graceful full-stocked rifle with good curly maple stock. But even if the gun were made by a son, it's still a Maupin rifle and worth acquiring due to your family's connection to Maupin. And if you really get lucky and it's an early, full-stocked rifle in good curly maple with 43" to 44" barrel, probably well-made but without patchbox or inlay work, I'd think 2,000 to 2,500 might be a reasonable price to pay.
Shelby Gallien