Guy,
I need to start by saying that I have no proof that rust bluing was used on mounts in America. It was used on English double barrels in the 18th century because their soft solder joints and ribs would not stand the heat of the various types of heat blue. I have seen European guns with what appear to be rust blued mounts and barrels.
We did a number of rust blued rifle barrels in the shop at CW while we were still trying to redevelop the charcoal bluing. We felt it was better than browning on an early rifle. My York Co. PA style rifle in Three Centuries has a rust blued barrel.
The rust blue is really simple because it is just like browning with one addition -- the part is boiled in water for a few minutes before each carding. As with browning, the smoothness of the finish depends on how well it is carded. There mounts were carded after fairly short rusting cycles and so vigorously that there was no detectable "drag" on the surface. This produces a hard, dense, smooth rust and the final application of hot wax makes it very resistant to additional rust.
Gary