Parts such as the Rear Cover: O Ring 23x2.8, Cap Tappet Hole, Oring,27.5mm, Bolt,flange,6x14 are included in the Cx500c Custom 1980 (a) England Rear Cover parts fiche.
The Nut Hex 4mm is the lowest priced item on this Rear Cover schematic.
The Rear Cover is made up of components including this Cover,rr. which is the most expensive.