"Ormolu"-style powder coating?
I don't know how it was done, but I have a very old percussion shotgun of uncertain origin, and it also has this brass effect on the very obviously hand-made lock. I think we need a metallurgist...
Just a guess... as my example is what one would generically term "Khyber Pass Special" from the 19th century, is can hardly have been created by modern powder coating or deposition techniques. However, it may have been achieved by painting the iron /steel parts with a brass paint (i.e. brass dust suspended in a lacquer) and then heating to bright red heat to fuse the brass to the steel. Something similar to the (very hazardous) 18th-19th C. ormolu technique for gilding.
That's all I can think of.:dunno: Any other suggestions...?