I cannot locate a washer that would be used on the rear brake mounting bolt, when the brake bridge is round. Looking at it from the side, the middle is thin and the radius will (hopefully) fit the radius of the brake bridge. This washer would prevent crushing the brake bridge by spreading the pressure around the bridge rather than along a thin line.

The bolt is probably an M4.

I cannot find one locally - does anyone have a source?

Or, does anyone have a couple they would be willing to part with?

Thanks in advance.