MAFAC RAID brakes top mount part?
I'm about 95% sure that is a proprietary part for this frame, not a Mafac part. Mafac adapter plates exist for standard orientation holes.

It appears that at some point someone added a wood screw in place of a proper bolt. I imagine the original intention was for this to be installed with a concave washer hugging either side of the bridge.

For what it's worth, RAIDs are a vastly better brake than R559s. If you're in a situation where a framebuilder level solution is needed to address the weirdness of the original mounting, rounding of the hole, etc, there's little point in adapting it to a modern caliper and buying a whole other brake when you could just get centerpull direct pivots added. You've already got the Herse/Compass tires, why go halfway?