I have no doubt these revolvers were "blued" or rather "blacked" by using the modern HOT "Bluing Salt" or AKA Hot Black Oxide treatment. The Plum Brown color can come from any of these things:
1. Contaminated or worn out solution in the tank.
2. Immersion of parts at the wrong temperature range.
3. Waiting too long after pulling the parts out of solution and before quickly immersing them in hot water and then rust preventative solution.
4. In the 1960's, Winchester made some Model 94 receivers out of steel that had some nickel in it or more than normal. They often to almost always come out "Plum Brown" in standard Black Oxide solution, however, the barrels stayed black. (These receivers come out Black in the Black Oxide for Stainless Steel Salts.) This happened to us and that is why we investigated it. We were going to "re-blue" the barreled receiver, but the owner liked it so much, he did not want us to do it.
Some folks have suggested it comes from improper cleaning of the metal before immersion in the tanks. Many people have attributed it to WD-40 not being cleaned off. However, in the huge number of guns we Hot Black Oxide treated guns over 20 years at Quantico, we found such improperly cleaned or contaminated guns took a spotty, sometimes wavy and generally not an even coat. It did not result in the Plum Brown color.
Around the mid 70's, we DID have one Smith and Wesson Revolver come out in that Plum Brown Color. I don't remember if the tanks had been allowed to cool down too far or were too hot, but it was due to improper temperature of the solution. The owner liked the color so much he chose not to re-do it. I saw that gun 10 years later and it was still a nice Plum Brown color.
Gus