Yes well done Mick, you've got it the right way round. Richard & Simon were almost there!
The normal operation of the Humber is as follows.
When engaging 1st gear, the gear stick is moved to the lower left.
Here it hits a protrusion on the 4WD lever knocking it down from horizontal, by about 20 degrees.
When moving up into 2nd gear, for normal roadwork the driver needs to return to 2WD by raising the 4WD lever to horizontal. In the stress of driving in NI this often didn't get done.
The gadget seeks to automatically disengage 4WD as soon as 2nd gear is selected. The spring loaded plunged is depressed by the 4WD lever & becomes held down by the slide latch. Once the gear stick is moved from 1st gear it releases the retaining latch. Whereupon the plunger pushes the 4WD lever up & into 2WD.
For the EMER sniffers, the Regulation was covered in EMER WHEELED VEHICLES N 257/2 Modification Instruction No.4 Feb 1976.
It would take 4 man hours to install the kit LV9/BOE/2520-99-821-8253