Top 63 brakes:
1. They are scarce, and it seems most ended up in Britain (if the e-bay seller nationalities are any indication). 2. They work as well as other Mafac brakes (very well indeed). 3. You can toe in the brake shoes without bending the arms. That is an appealing feature. 4. When used with brazed-on pivots, you lose the height adjustment for the pads, so the pivots need to be placed exactly right. That is why the French constructeurs didn't use them much, if at all. I have seen them on photos of Toeis et al.
I think 1 and 3 combine to make them desirable.
Jan Heine, Seattle