I'm sorry to report, after careful inspection, that my Compur Rapid has no such fitting anywhere in and around the lens /shutter. It simply doesn’t exist.
I think I read another poster saying the same thing about his Agfa Billy Record II. He was upset because he couldn't use a cable release.
Jurgen told me not to use the self timer as well, because the little knob breaking is the most common reason that he ends up seeing the cameras again.