On this parts fiche the Washer,6.5x20x1.6 is the most specified component.
The Taillight ('80-'81 Ke175-d2/d3) assembly is constructed from several parts from which the Tail Lamp Unit is the most expensive.
Some items from this specific schematic: Collar,2.5x5x9.8, Washer-spring,6mm, Collar-battery Case, Bulb,6v-32x3cp.