I'm 100% convinced the iron mounts on these guns had no metal finish of any kind and were left in the white. The surface finish could range from rough file marks to a fairly smooth polish. I own an original with the metal finished in what I describe as "polish over file marks". In other words, it has a pretty smooth polished surface that took some time to do... but no attempt was ever made to remove all the deeper file marks. This runs counter to what we're told to do in all the books on gunmaking.
The color we see today is nothing more than the patina acquired through age. I have old tools from my dad with the same color. Another plausible idea is that the hardware was left in place when the guns were stained with aqua fortis. The acid would have been wiped on metal and wood at the same time. This is plausible when iron hardware was nailed in place. I can't believe they temporarily attached the hardware... removed it to finish the wood... then nailed it back in place. They would have simply nailed it on as they were building and left it alone.
Artificially aged guns are very popular today. The closest one to new-original looking that I've seen was this one by David Rase...
https://americanlongrifles.org/forum/index.php?topic=55387.msg554073#msg554073He taught a class on this. You can apply a standard rust brown as per the instructions, then wipe it off using Naval Jelly and steel wool. It's that simple. Of course... after doing all the work to get a good rust brown finish, it's really hard to convince yourself to strip it all off.