" I just looked in the likely room, but don't see it. I will keep looking. "I guess I will chime in here. I have a background in military archaeology, so this is the sort of thing I think about.
A military person would be expected to keep their gear in good service, which included their brass. There is plenty of evidence of this, especially from period correspondence.
I have a lot more old books than most other people, and there is one in particular that should answer this if I can find it. I just looked in the likely room, but don't see it. I will keep looking.
In the mean time, I just looked at a bunch of art depicting frontiersmen and pioneers, painted more or less contemporaneously to the time portrayed. What I saw was reasonably shiny, which is what I expected.
The Japanese have always been the best source I know for traditional coloring and patination of metals. But even a well darkened piece will become shiny again with heavy handling.
I do not believe that it was typical to use the sort of darkening processes common in Japan because it requires a great deal of work, and some practical chemistry. If our frontiersmen did this, they would have mentioned it. It is no trivial matter.
I expect that the polish used would typically be a mix of flour and vinegar, with a bit of fine salt added if needed. It would then be oiled. Brass or silver on a gun might not be kept military clean in the field, but I cannot see anyone whose lifestyle relied on such an expensive object to allow it to corrode.
The biggest room in our home must be called "someplace safe". It is chock full of things we can not find.