PDD-
Hot bluing refers to the more modern process of immersing parts in liquified salts at temps over 300 degrees. Rust bluing involves coating barrels with multiple thin layers of red rust and boiling them between carding sessions. Rust bluing is the traditional method used for bluing soft soldered doubles that can not withstand the temps of the hot bluing process, as boiling water is only 212 degress F. But everything I've read indicates that venting of the air trapped between the top and bottom ribs is necessary to keep the pressure from popping a rib loose during the boilings. But Parker apparently didn't do that at the factory, as original condition guns don't have any vent holes that I have been able to locate. Perhaps the sighting bead hole on Parkers doubled as their pressure vent during the bluing process? I don't know. Hopefully someone else will and share that knowledge.
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