Bill,
Great question. A few things helped attribute it to the McInturffs. The “J.M.” barrel marking is identical to the marking on the John McInturff attributed rifle on page 56 of Dave Byrd’s “Gunmakers of Buffalo Valley and Greasy Cove in Unicoi Co TN”. I actually have a forged barrel that is marked this way as well. Also, the architecture and mountings are nearly identical to the one known signed Christopher McInturff rifle and the early iron mounted Laban McInturff rifles. Lastly, Dave Byrd examined it and gave the attribution as well. Sorry for my extremely late reply!
-Adam