Author: B. Cunha
The early MGB MKI equipped with the D type overdrive unit used a firewall mounted, vacuum activated switch as a part of the overdrive system. The switch ran off manifold vacuum to activate the switch at around 7+ inches of mercury.
EDIT – Bruce Cunha has a beautiful webpage that not only provides a pictoral of the vacuum switch but also provides guidance on operation as well as calibration.
The website may be found here:
I (M. Feig) am showing front and back pics from one of my OD Vacuum Switches for illustrative purposes only.