I can verify that it is a Schmidt coupling, I worked on many of them on German made envelope making machines. Even though they were turning at one hell of a rate, I never came across one that failed. The ones we used were the three plate design, as in the video. They allowed major backwards and forwards adjustment of say a cutting knife without having to stop the machine, just wind the handle until the cutting knife lined up again, the coupling took up the out of alignment problems.
One coupling that is not mentioned much is a Hobson coupling, even though all the elbow engine builders have made one (sometimes not too successfully). That isn't designed for making an engine out of, but to be able to rotate the 90 degree output around a 360 degree swing. It just so happens that when you put pressure on the ends of the right angle pins (pistons), under certain favourable circumstances, it will turn all by itself.
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