Well, not for Parkers, but I do have the factory records for Winchester circa 1898. Every part…pin screw etc.
During that period, the major finishing processes were: Rust bluing, Charcoal Bluing, Nitre/Heat Bluing and Case Hardening.
Keep in mind, that these finishes were not unique to the gun trade, but were “Industrial Finishes”. Look at some period tooling; Squares, Taps/dies, clamps, V-blocks etc., some have beautiful CCH finishes. Clocks/Watches, dinnerware with Nitre blue finishes. The arms on Royal Typewrites and the bicycle chains on Schwinn bicycles; Carbonia blue.
OBTW, Carbonia Blue didn’t come out until 1903 or so, a proprietary process by the American Oil & Gas Furnace co.
Years ago, I wrote an article for the Winchester Collector detailing these finishes.
As to the OP question, an educated guess is that if the part was originally blued, it would have been charcoal blued. Most screws on Winchesters (again not Parker) during that time frame were Case Hardened. Manufacturers that made a lot of screws use “Screw stock” similar to today’s 12L14, it’s softer than regular steel, machines nicely especially in screw machines (saving wear on tooling); but is more easily damaged, so they would case harden them to provide a hard outer layer.
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