OP,
That’s one fiiiiiiiiiiiine looking revolver!
Is it possible the S&W Custom Shop made it? I thought Smolts had the usual Colt barrel markings? But I have no idea what they are.
I have a full lug S&W60 3” in stainless, FWIW.
ETA: Massad Ayoob used to have a Lethal Force Institute in Dunbarton, or Concord, NH. If he’s still around, I bet he could help ID it. He used tweaked revolvers before he went all semiauto. I had emailed him in the past & he was always helpful. Roy Jinks might be another option as the S&W Historian, but I don’t know if he’s still around.
https://www.smithandwessonforums.com/forum/s-w-revolvers-1945-present/93529-k-frame-revolver-full-underlug-if-not-2.html