Those are both beautiful revolvers! I like the smooth target stocks on the M15. The no dash M66's are not that common-and to find one "Unfired" is pretty uncommon!
First, determine why it's turning dull. Standard finish is a matte stainless finish or what some folks call a "frosted" stainless finish. This is a non-reflective finish S&W put on their earlier stainless guns.
If the gun is used a lot and oil used to remove powder residue, it can turn dull. If the gun still has it's frosted finish in good condition, then strip the gun down, degrease it thoroughly, then clean with a good powder solvent and a clean nylon brush, keeping the gun wet. Rinse with solvent or warm water and dry. Reassemble and lubricate.
Before embarking on polishing job using Mothers Mag & Aluminum polish you should know that it will remove the "frosted" or matte finish. Once you start, you'll be committing to polishing the whole gun. If you polish with Mother's it'll look like bare stainless steel. Finishing with a fine stainless polish can make the finish shine like a nickel finish, if that's desirable to you.