What is this pneumatic switch/valve ?

Car = 1986 300E , UK model.

Located in the battery compartment (right hand drive vehicle, could be different on an LHD) on the firewall is a small switch or valve type device with two pneumatic tubes attached and a plug connector with two wires.
One of the tubes goes back thru the firewall , down behind the engine into the vicinity of the trans. The wiring just runs into the main harness.

Can some1 tell me what this does please , I only have limited info on this vehicle and cannot find reference to this item. I suspect it has something to do with the cruise control system.

Thanx in advance.