You know, when Dad was on PhxPD, he carried a stainless S&W M60 .38 snubbie as a back-up gun. And because he was on a walking beat in the Deuce, he sweated all over that thing, which is why he bought the stainless steel gun in the first place. It didn't take him long to stain that "stainless" revolver.
It looks like this was an early Model 686 when the frame, cylinder and barrel were stainless but the lockwork was still carbon steel. I'm almost willing to bet something similar happened.
