Early fusebox wiring

Just putting mine back together, have the early type fusebox with the bullet fuses, theres a wire on connector D on back, pin 4, but cant find any info on what this is for? Anybody know?

Thanks

All of my saved diagrams show this pin empty, its linked internally to A/2 and A/2 has a blue/brown or blue/red wire with an unused plug.  If I had to guess, I would say either diesel glowplug and/or choke warning light related