The O-ring,2.5x1.5 is the lowest priced item on this Rear Brake Caliper (st1100a) schematic.
The sub assemblies and components that make up this Rear Brake Caliper (st1100a) include this Caliper Sub Assy,, the most expensive part.
A sample from the products for the Rear Brake Caliper (st1100a) include: Caliper Sub Assy,, Pin,bolt, Piston,27 A L, Retainer,brkt Rr.