single pivot was last seen in the campag catalogue in 2018. If one of these then a cone spanner on the central nut (that the spring mounts to) as LWaB suggests.
Things that can stop the brake from centring include
- that the centre nut is poorly adjusted
- that the cable housing is pulling the brake to one side
- that the centre pivot is binding
- that the spring is binding (instead of sliding easily) where it bears against each arm
The last of these is surprisingly common; seasoned bike fettlers habitually add a tiny drop of oil to each spring contact whenever they have the can in their hand.
If the centre pivot is binding, instead of taking it apart, it may be sufficient just to douse it in oil and 'exercise' it.
cheers