British shotgun and rifle trigger guards, top levers, and floorplates were not nitre blued; they were charcoal blued, a very different process. Nitre blue is a form of "temper" blueing, where the temperature of the metal to be blued is controlled by a molten sodium/ potassium nitrate [nitre] bath. This results in a very bright 'fire' blue, or a purpleish blue. A charcoal blue is similar to a color case where the parts are heated in powdered charcoal. The colors are very different, and the wear qualities are as well. A charcoal blue will wear much longer than a nitre blue. Doug Turnbul's "carbonna" blue is very close to a British charcoal blue, but I have noticed a slight color difference.
I hope this helps, Mal
