There were and they were shown in the early manual, but no one that I know of has ever seen one in the flesh(35+) years. It was a piece of angle tron that attached to the side rails and had levers down the length that when turned 90 degrees would lock the stirrups of the litter to keep it from moving. The levers looked just like the ones on the fixture that you strapped to a litter to hold the "Thomas Ring splint" in place. There was another version that used pins.
I seriously doubt this was put into production because that model mule wasn’t. Look at the sloped foot basket and the angled steering gearbox. My guess is that this photo is a pre-production prototype.
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