The Nut.hex 10mm is the cheapest item from this partsfiche.
Parts such as the Rear Wheel: Collar,rear Wheel, Balancer,heel(20g, Tube375/400-18tr6, Damper,rear Wheel are included in the Cb550k3 Four Australia Rear Wheel parts fiche.
The thirty-one Cb550k3 Four Australia Rear Wheel component parts are all listed in this schematic.