What is the name of this part/mount/adapter that mounts an old school center pull brake to the top of a rear bridge? This is a 1970s MAFAC RAID on a LeJeune Tandem. I'm trying to find an adapter to mount a modern Tektro R559 Long Reach instead, thanks.
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?
I wonder if frame builder could "fill in" that hole with braze, and then drill a new brake hole horizontally-ish for a more modern brake?
If there's not enough meat, or if the brake bridge is too high then the frame may need a replacement brake bridge brazed on in a more-conventional place slightly lower down.
A third option might be to rotate that "hook" bit and bolt it to the existing brake bridge from below, gaining you a few millimetres of reach. Do use a proper bolt, that screw is not the right fastener.
Most complex might be to engineer up a whole block that clamps and bolts through that vertical hole, and provides a new horizontal hole for the brake. Again this could lower the new brake closer to the wheel.