300 tdi, What is this part?

Hi every body, i don't normaly post just look at all the great guide and things.

I have a Baby 97 defender 90, and i was looking at the engine and it looks to be a bit missing, or a bit taken off or not needed.
it's to the left of the block, the screw threads look used and clean... ish
I don't have a working camera, so i pulled this image of wiki, this image has a the part fitted, i was wondering; What it is? & What does it do? & Should i get one?




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air con pump as yours doesnt have air con you have no pump ,nothing unusual

could i use the the air con pump for compressed air?

yes but need some other parts