I personally much prefer good close grain walnut for gun stock wood compared to maple but am curios as to which species was more prevalent and available to the areas you mentioned that birthed the SMR. Iron furniture makes sense as that was the least expensive available material but I'm wondering if walnut was that much more redily available than was maple. I've read cherry was also fairly popular gun stock wood.I have to say, some posts have a lot of misinformation in this thread.
Eastern TN rifles and Western NC rifles had very similar features. Walnut stocks were on 95.5% of them with a few maple. Maybe 1 out of a 100 were other woods. Early gunsmiths supplied the Scotch-Irish families who settled the Appalachians starting in the 1740s and ending during the Revolution. Were long rifles made in TN/NC that early? Probably, but also a lot were brought down from more settled colonies.
But by about 1800 or 1810, there were gunsmiths in the back hills, forging the native iron and building rifles to fight Indians, hunt game (deer, wild boar, turkey) and a generation later, fight bushwackers. It was wilderness.
The Piedmont rifles are fancy with lots of brass and ornamentation because their users lived in towns or on farms near towns, trotting around on fine horses between villages on plank roads. In 1820 Appalachia, there were no towns or roads...it was rough frontier. In the Piedmont, there was no longer a worry about Indians by 1810, but you still had lots of bad guys in the mountain border regions. Just like today, the government expected the border settlers to be a "buffer" against all that bad stuff, and protect the landed gentry down in the flatlands.
Appalachian mountain rifles have a distinct style because of their isolation and resources. Iron ore was around, and used instead of brass which was expensive. Walnut instead of maple. Grease holes instead of beautiful, fancy brass patch boxes.
The styles we study as sounthern mountain started in middle NC, (which were a merging of several Northern styles like Moravian and Lancaster), then moved westward into the foothills counties like Catawba, then further into the mountains of NC, then over the Blue Ridge into eastern TN. In that direction. East to West, from the very late 1700s to the 1830s. Once the styles were established in the counties of the far Western NC, they carried over (literally) into Tennessee. The much earlier, Revolutionary TN "over the mountain boys" surely had Northern long rifles traders brought down to them. It was too frontier in 1776 to even have blacksmiths and such, for the most part.