The OMM Angle Brackets are used for mounting OMM racks and attaching the extenders to the V/Cantilever brakes. On bikes with disc brakes that also have V/Cantilever brakes bosses on the frame the larger diameter angle brackets are used to attach the extenders directly to the V/Cantilever brake bosses.
The Angle Brackets comes in three sizes: 6mm, 8mm and 10mm. The 6mm size is the standard size that is included with all OMM racks. The 8mm and 10mm are used on bikes with disc brakes that still have available V/Cantilever brake bosses. Brake bosses are either 8mm or 10mm, 8mm being the more common size.