Originally the Colt magazines were tempered, and then heat blued. The heat from the bluing process was removing the temper from the top of the magazine, so Colt started bluing the magazines, and then tempering the magazines by dipping the upper part in molten cyanide at around 1600f degrees and then quenching in oil. This process removed the blue where the magazine was dipped. When the bluing/tempering process was changed, all of Colts pistol magazines from .22 to .45 were done this way up until 1940.