On this parts fiche the Nut,u,10mm is the most specified component.
four components are shown in this Vt600cd Shadow Vlx Deluxe 2002 (2) California Rear Shock Absorber exploded diagram.
The most reasonably priced component used on the Vt600cd Shadow Vlx Deluxe 2002 (2) California Rear Shock Absorber is the Nut,u,10mm.