Here are the results of my tests on the 1986 to 1989 930 distributor, P/N 0 237 302 045 with PGFU6 ignition control capsule.
For vacuum applied to the front and not the back advance occurs
For vacuum applied to the front and the back advance occurs
For vacuum applied to the back and not the front no advance occurs
Here are pictures of my tests with 1 bar pressure:
