Not sure on the SD but the switchover (or overboost protection) valve is mounted to the firewall, behind the brake booster on the W123s. Plumb a plastic tee in there and snake a line into the cabin and hook it to a 0-30 psi air gauge.
Here's how I made one (look at the 10th or 11th post).
FWIW, my SDL has a similar-looking valve that is the cold shift valve so, don't hook to the wrong thing on yer SD.