Here is a picture of the internal set screw on the bimetal arm inside the WUR, from my 82 SC. The part number is 0 438 140 090, and it has one vacuum connection at the top that goes to the throttle body, but nothing at the base near the main spring.
The set screw and electrical contact are actually on a second, smaller bimetallic arm.
You can also see in the yellow circle that there is an electrical contact switch, which I believe interrupts the heater circuit once the bimetallic arm has bent fully. Perhaps the set screw is to adjust this temperature point and therefore the warm control pressure.
Can anyone explain how this screw would affect the operation of the WUR?
Olivier