OK another theory from Geezer - the vacuum switch disables the ignition coil when there is zero vacuum available so it might be (for some bizarre reason...) there to kill the coil to prevent the run on issue after the ignition is killed (still don't know why it would even need it). The ignition advance servo is on the distributor so that is a different control peripheral. If it's running on afterwards, check your ignition timing and check your plugs to see if it's running rich. The run-on is not good for the engine. Gen 1's came in 2 load ratings - 500kgs and 1000kgs. The 500kg rated trucks are easy enough to identify - just look at the tail/drive shaft. If it looks like the diameter of the shaft is on the thin side, it'll be a 500kg truck. AFAIK the differences between the 2 rated load capacities are the drive shaft, the front stabiliser bar and the upper control arm pivots. Not sure of the spring ratings but it would be logical that there would be an uprated spring on the 1000kg trucks.