Interesting discussion. When I was a young'n, the church we attended, in New Monmouth, NJ had, amongst it's membership, families that predated the American Revolution. Families that had ancestors that were fairly substantial landowners and farmers, in that period. They were still substantial landowners and farming in the early 1950's. One of the sons, knowing of our interest in hunting, firearms and 'old guns, invited my father, one Sunday after church, to bring us over to see their 'old guns.' From a hidden compartment, built into a paneled wall and tucked behind a high-back arm chair, he brought out, one at a time, two (2) Charlesville - US - marked muskets, brought home by two brothers that had fought with Washington, a fine early Lancaster (daisy patchbox and all) and then a longrifle, he called a 'plain gun' that was the familys' "earliest" piece, as he was pulling it out. He only showed us only one side (lock side,) of this piece, but it had an absolutely stunning piece of tight curly maple, from butt to nose, covered in a beautiful reddish varnish finish, a long swamped barrel with sights, a fairly large flat Germanic/'Lancaster' style lock (not a banana shape, that I can remember) a straight combed stock... with a single 'round' hole in the buttstock. I can't tell you about the particulars of buttplate (if any,) ramrod pipes, nosecap (if any,) sights or rifling, but I can tell you that my brother and I stood there with our mouths open. I mean, here was a longrifle that challenged all of 'our' preconceived notions of what a longrifle was, from information garnered from collectors books that we could get our hands on and longrifles in museums. It was a "WOW" moment and I can still see it in my minds eye. The metal, by-the-way, was still fairly bright. That was the only time that piece came out and I never saw it again. I did inquire after it, last year, through my father and was told that it still remains with a family member (brother,) who now lives in VA.
I submit, that there was nothing about this piece, that would make one think that it had ever hung in a barn, was a parts gun or wasn't finished by a professional hand. And here, a linear ancestor, referred to it only as a 'plain' gun. I'm also quite sure that a hole drilled in the buttstock, for patch lube... isn't confined to just 'southern' pieces. I believe, that a professional gunsmith, like a Newcomer, a Beck and others, were capable of producing whatever their customers wanted, perhaps even incorporating a feature from another part of the Colonies, into the prevailing regional 'school,' if that's what they wanted. Given that the colonists followed and kept up with clothing fashions in Europe, I don't think our ancestors where quite as isolated, from each other, here in the American Colonies, as early historians portrayed.
Subject to the known facts and absent anything which might prove otherwise, the term 'plain rifle' or 'plain gun' (another topic on the board) may have, in fact, encompassed everthing from the so-called 'Schimmel/barn gun' to a piece with some incised decoration behind the cheekpiece (maybe) and a hole in the buttstock for patch lube. The answer, if any, more aptly lies, between pure conjecture and pure semantics... of our own invention
Just throwing my stone in the pond, here.