Parts such as the Taillight: Bulb (12v5w), Nut,flange,6mm, Washer,4.2mm, Base Comp.,tailli are included in the Vt1100c Shadow 1988 (j) France / Kph Yb Taillight parts fiche.
In this schematic the "Nut,flange,6mm" is used most often.
The Screw,tap.4x30 is the cheapest item from this partsfiche.