The Washer, Special 10.5x21x2 is the lowest priced item on this 10a - Half-crankcases Pair schematic.
A sample from the products for the 10a - Half-crankcases Pair include: Nut, Flange M10x1, Half-bearing Green, O-ring, Gasket, Aluminium 12x16x1.5.
On this parts fiche the Screw is the most specified component.