70 series altitude compensator - advice needed!

[SAFARI CRUISER]

Greetings to fellow Land Cruiser fundi's.
I've just bought a second-hand 70 series pick-up (JTELB71J1070XXXXX) with 1HZ diesel in South Africa which I will be using in the bush of Mozambique.
I see a small black plastic knob on top of what I believe is called the ALTITUDE COMPENSATOR. It rotates about 270 degrees. See photo.
The owners manual does not mention it at all (dealer could have provided the wrong manual).
Can anyone advise what is the purpose/function of this knob? And suggested position for low altitude operation?
THANKS. I'm new to the site and look forward to learning a lot more about this rig!
Cheers

 

[PhrogsPhorever]

That is a rubber hood/cap for the altitude compensator bellows, not a switch IIRC

 

[ForealBoreal]

you have one of the pumps that has a boost comp built in.
Lucky.

 

[SAFARI CRUISER]

Thanks guys . . mystery solved.

Irritatingly enough, I had the bonnet up the other day and now the damn thing has gone missing!
As the cap appears to have a 'pointer' I figured it was a way of making some adjustment. But now that I see it removed, it is indeed just a cover for a tapered hollow tube (which of course does not rotate). It must be a fairly uncommon installation as I've not seen it despite being somewhat intimate with numerous 70 series machines in Africa over the years.