There are two models known to me, the WL63AMR/FA/2 and the WL63AMR/FA/3. The instruments look identical, but the connector differs as shown below. The left one is the WL63AMR/FA/2 model and the right one is the WL63AMR/FA/3 model.
There's one heavy duty circular connector at the rear. Luckily Michel found out the connection for use for the WL63AMR/FA/2! The connections are mentioned below.
A: 26 VAC 400 Hz power supply input
C+D: 5 Vrms for dial lighting
error flag indicator:
E+J: +28 V for the arror flag actuator (signal on = flag removed)
G: 0...10V input voltage for 0...5000 feet indication
H: measurement reference voltage (Vref = 12V for 0...10V indication)
F: common terminal for input [G] and reference voltage [H]
Info yet unknown...