The Washer-plain,5mm is the lowest priced item on this Taillight (-'72-'73 G5-a) schematic.
On this parts fiche the Washer-spring,6mm is the most specified component.
Some items from this specific schematic: Bulb 6v 3/21cp, Damper Rubber, Bracket-reflector, Washer,8.5x20x1.6.