D C Plumber
That's kind of what I thought when I first saw the initial photos of your gun. Colt's satin nickel looks like the metal surfaces were bead blasted first, giving it a decidely matte or frosted look. Your gun looks like it was polished first, then had the electroless nickel applied. To me it gives the finish a more even appearance; looking more like stainless steel.
Whether all this occured at the factory at the time it was built, later after being sent back to Colt, or if the work was done by someone else, is open to speculation. I think I would still get the factory letter, just to see what Colt can tell you about the original finish on the gun.