Just trying to get a handle on it. As I understand (and that is questionable) the Moravian gun makers were a big part of the American gun making history. They immigrated, initially started in the PA area, and subsequently migrated down the Shenandoah valley, ending up in SW VA, western NC, and even into what is now NE TN. People were apprenticed and moved on to new territories. The "VA" rifles looked very similar to the PA rifles. Yet somehow, in the space of a few years, the people who apprenticed under builders in VA and PA, started building a rifle that had a very different look when the moved to western NC and northeast TN. How did this happen?
Your introductory sentences cover a lot of territory and time. Roughly from the middle of the 18th century to the third or fourth decade of the 19th century or 70 to 80 years. There were a lot of changes that occurred in American rifles during this period. And several different generations of gun makers and customers that didn't necessarily want a rifle just like Dad's and certainly not like Grandpa's.
I think you would be hard pressed to find a SMR or TN rifle in classic form that dates to the 1820s. Some date to the 1830s, but most are likely from the 40s and 50s. By that time, the trend in longrifles in a number of regions were slim, light rifles. If you compare an SMR to a Leman rifle or a Western PA rifle or an Ohio rifle of the same period, you will see very similar stock architecture with narrow butt plates and significant curvature to the crescent of the plate. The SMR stands out in its mounts of forged iron, but not much else.
The idea that the American Longrifle "evolved" is not a helpful concept. It's not a living organism so it doesn't follow Darwin's Theory of Evolution. It's probably better to think of it in terms of technical and artistic advancement. I know of no examples that one can point to that shows a gradual, linear progression from one type to another. Technology, and to some extent art and fashion trends, follow fits and starts and leaps and bounds. Looking for evidence of the evolution of the SMR is akin to looking for a unicorn. You aren't going to find it.
The butt stock on the rifle above, which is attributed to Baxter Bean, has the classic triangular shape of a Lancaster rifle. Compare it to the lines of the two Leman rifles below from about the same period. Again, the biggest difference between the rifle above and the rifles below are the mounts.
There is no evidence of scarcity of brass or that an abundance of iron led to the iron mounted rifle style. I cannot imagine a scenario where they could get locks but not brass hardware. And they did get locks.
I agree with Rich. If the Southern Mountain gun makers wanted brass, they could have gotten brass. As he says, they imported almost all the locks they used from England.
Pennsylvania produced more iron than the Southern colonies and states and began producing it as early as 1716. Between 1810 and 1840, Pennsylvania consistently produced half of the national total. If it were purely a matter of economics and availability, why weren't Pennsylvania longrifles mounted in iron instead of brass.
The iron mounts of SMR's and TN rifles developed not out of necessity or economics but from artistic expression and as fashion statements.