The most reasonably priced component used on the Gsxr1100 1990 (l) (e01 E02 E04 15 16 17 18 21 22 24 25 34 39 53) Tail Lamp-license Lamp (see Note) is the Lock Washer.
On this parts fiche the Bolt is the most specified component.
Some items from this specific schematic: Cap Od:40, Gasket,license Lamp (nas), Cushion, Bulb(12v5w,bf150-40160a).