Wasn't charcoal or carbona bluing more of a standard finish on guns right before and right after WW2 by predominately American companies? I thought most British furniture was blacked using the rust bluing method. I would say most all the vintage Brit doubles I own have rust blued furniture.
When Brits "refresh" a double gun, they often like to nitre blue the screws and pins, not all the time, but I've seen enough of them done like that to form an opinion.
Here is a Cogswell & Harrison that has its furniture Carbona blued and the screws Nitre blued. If I had to do it all over again, I would've had the furniture rust-blued instead. The Carbona blue is too shiny, looks too much like hot blue for my taste. I prefer the soft-velvety sheen of properly polished rust blue.
