I remember the 617-5 (6-shot, MIM, IL) usually coming with a .500"-wide target hammer. The -6s (10-shot, MIM, IL) usually ship with the .375"-wide semi-target hammer, but the target hammers from the -5s should fit.
S&W lists it as part # 293660000 ("HAMMER ASSY .500 (STIRRUP NOT INCL.)") in their Revolver Parts List
. There's another .500"-wide MIM target hammer listed, part # 228080000, but that's for the -4, which was MIM but pre-lock (which could work in a -6 if the lock were removed).