Electrical System Diagnostic Plug / Test Port

[grumble]

I’d contact Les. 
Reckon he would be the only one that knows the answe. 

As far as I know it’s useless

Barry the diagnostic plug actually had a connector that we connected when servicing and allowed us to test various circuits including the dwell, set the timing etc. I left the dealership to work for the service manager at a new workshop at Tuncurry specialising in electronic tuning and wheel aligning shortly after the unit was fitted so I never had a lot to do with it. If you use a test light you can work out which terminals belong to the tail lights, brake lights, dwell etc. I am certain that the designation is in one of the manuals but I would not call it useless.