Larry, I was chatting this morning with a local MB tech and asked him about the scene you described. I knew he had an experience with a runaway Mercedes diesel.
He said he was working at a dealer in New York in 1978 and one of the techs replaced the vacuum shutoff on a 123. None of them were aware of the potential for a runaway engine at that time, and so no precautions were taken.
He said the tech started the car and then walked over and began filling out some paperwork. He said the engine started to slowly build up speed, and by the time the tech paid heed the engine was running at a very high RPM.
He thought there would have been enough time there early on to have pushed the stop lever in and effectively shut down the running, but the delay by the tech prevented that.
Is that what you saw, or did the one you have just take off out of control right off?