FMS 20 is for mod OP 07-1487, which is power take off for Abex pump. The Abex pump is only for the second dual hydraulic pump, according to the Index of Modifications book. The mod number is quite old, and predates the introduction of the B3 with dual hydraulics, so I wonder if Airbus had drawn up the plans for this gearbox around the same time they created the twinstar, and just not utilized it.