AFAIK they used a single pin differential pressure switch for late HG, all HQ & HJ. HX saw the 2 pin switch & this carried thru to HZ & WB.

On the late HG & HQ the switch was mounted in a junction block. There was no proportioning valve on HGs & on HQs the prop valve was mounted along the subframe rail under the driver's seat, on disc braked cars only.

For HJ, HX & HZ the switch was mounted on the proportioning valve. In WB it was mounted on the master cyl which also had the proportioning valve in built.

Dr Terry