Parts such as the Rear Master Cylinder (gs750c: Cap, Bolt, Upper, Washer, Piston/cup Set are included in the Gs750n 1978 (c) Usa (e03) Rear Master Cylinder (gs750c parts fiche.
The Nut is the lowest priced item on this Rear Master Cylinder (gs750c schematic.
On this parts fiche the Nut is the most specified component.