Apparently, when Rollei developed the focal plane shutter for the SL66, they had focal plane shutter tester called the EKZM.
When they later developed the SL35 focal plane shutter 35mm camera, there was an adapter plate to use the EKZM shutter tester on the SL35 camera.
The service specifications for the Rollei SL35 shutter indicated a transit time of 10.9 to 11.4 milliseconds on the EKZM. However, without an EKZM tester, one is left without knowing the transit distance over which the time is relavent. For example, the Pentax literature (below) clearly shows the the sensors of their shutter tester to be 32mm apart.
I was able to find some pictures of a Rollei EKZM shutter tester on the internet, along with some description of its function.
Elektronischer KurzZeitMesser (Electronic Short Times Meter) from 1968.
I'm guessing it works by charging capacitors with a photocell and measuring the charge on the capacitor with the galvometers on the front of the device. With the charge on the capacitor proportional to the exposure.
The photograph allowed me to determine the distance between the sensors. The distance between the sensors being 35mm based on a known film gate size of 36mm.