Rework
iby,
I agree with Mike,
Also, the Nitre or "fire bluing" on the screws is easy to restore or replicate. The process is simple. Resurface, clean, apply bluing, insert into blue flame until the "Fire bluing" turns the screw head this fabulous blue, pull out of the flame, then dip into oil while hot. That's exactly how they turn out. I've done it many times on restorations of nickel guns. The only trick is when to pull out of the flame and when to dip into oil for the best result.
The photos depict a bluing applied "over rust" all over the frame and barrel. That's exactly what it looks like. Rust is rust. It doesn't ever match the bluing. You can heat it, oil it, wipe it, sing to it, but it never turns blue. Only if you apply hot bluing over the rust will it turn blue like that.
There are different forms of restoration depending on the original condition of the gun to begin with. This gun in my opinion had spots of rust and only patches of original finish. So the person simply cleaned and lightly polished the surface and then applied a hot bluing leaving patches of rust still on the gun that worsened with time and started flaking off. That's exactly what you are seeing with this antique. It looks very nice but it's a restoration.
Murph