The Nut,6mm is the lowest priced item on this Taillight G4tr-c ('70-'73) schematic.
Parts such as the Taillight G4tr-c ('70-'73): Body-comp-tail Lamp, Bolt-upset, Bolt 6x20, Collar,6.8x10x8.5 are included in the Kv100-a7 1976 Usa California Taillight G4tr-c ('70-'73) parts fiche.
In this schematic the "Washer,6.5x20x1.6" is used most often.