Well I don't know much about British guns, but I can talk a bit on American guns, specifically Winchesters.
Winchester didn't start hot bluing their guns until 1939, salts were supplied by Du-Lite.
Prior to 1939, frames and other "smallish" parts were Machine AKA Carbonia Blued, That started around 1903. Process supplied by the American Gas Furnace Co. barrels & mag tubes were rust blued.
Prior to machine bluing, Winchester Charcoal Blued frames and larger components: bolts, buttplates etc, Smaller components were nitre blued.
