Nice job taking it apart and in the cleaning. When the extension rib was put on from the factory it was brazed on, so what you are seeing is residue.
Your friends gun was made Jan. 1901.
To remove rust from the barrels, sand them as they will have to be rust blued if steel or if Damascus will have to be rusted and then etched.
The gap between the water table and the barrels is good, there has to be a gap for wear. If metal to metal that is no good.
Glad to see that you got the rotary bolt together, the coupler pin that goes into the rotary bolt has to go into the side where that hole is in the rotary bolt or else the gun won't open.
Compressing the top lever spring could be hard to do unless you know a way to do it. If you have problems let me know.
