Mystery part for VW Golf Mk1 Rabbit (Cabriolet) engine

Hi, my brother is restoring an Mk1 Golf Cabriolet (Rabbit) in the US.

He has a two-pin plug that he doesn't quite know where it goes – but found another Mk1 Golf that has that plug connected to what looks like a circular part (valve?).  I've attached a photo below.  Can anyone tell me what this is?

Thank you!

Tony
  

Upshift switch for the upshift light in the center top of your clocks that I have known, but the UK may have used it for the MFA.

Thank you!!  That makes sense – so the 2-pin connector must then go to the upshift light on the instrument panel then.  

It is a vacuum operated switch.