OK Drewbie- here we go-- May/June 2020 issue of SS magazine Fine Gunmaking by Stephen Dodd Hughes-- on the "Perfect Repeater" :: page 36 mid-page graph- quoted: "Bluing loss is pretty typical for M-12's, and that goes along with another theory of mine. I think the so-called nickel used in these guns doesn't hold its bluing as well because of the alloy of the steel (add: my obs. try bluing chromium, whether rust bluing or the later dip-tank process)-- "According to Tool Steel Simplified-- Nickel does add to the toughness and wear resistance when used in conjunction with hardening alloys like chromium. Prior to about 1939, M-12's were rust blued. For the most part, the bluing holds up well on the barrels but seems thinner and to wear faster on the harder steel of the actions and barrel extensions"!!!
I also wonder, as Dad's 1938 era M21-12 gauge Field grade shows a browning shade on the receiver floor plate, but no the graying shades of finish wear on the barrels (it has a splinter field style forearm) and has been shot and used afield a great deal since he bought it secondhand in 1948, gave it me in 1980 (Thanks, Dad)
The oldest of my 7 Model 12's had a nickel steel barrel. A field grade 12- 28" barrel Full Choke sn.264437 possibly mfgd. in 1921- the barrel, mag tube and barrel extension shows good even solid blue, but both sides of the receiver, also top and part of the trigger guard are a worn with careful usage a grayish hue- even, NO rust anywhere (true with all my other shotguns as well.
I still tend to beleieve that WPS was WRA's marketing move, following on the heels of the Stainless Steel barrels offered in the late 1920's era.. RWTF