On this parts fiche the Washer,oil Bolt is the most specified component.
A sample from the products for the Rear Brake Mastercylinder (cb600f/f3) include: Diaphragh, Cup,mas.cylin.oil, Circlip,m/c, Nut,hex.,8mm.
The component with the lowest price in the REAR BRAKE MASTERCYLINDER (CB600F/F3) schematic is the Screw-washer,4x12.