I have finally and conclusively found the source of the missing light. 

After suspecting a faulty aperture, Thorium in the lens (which is there, btw), unpredictable film stock, incorrect light metering, nearly buying a light meter… I can finally prove that the problem is the shutter!

Unfortunately, the shutter is electronically controlled.  Electronic control in a camera from 1969…  now how on earth do I fix that?