Used: Rollei FRC1 Foot switch for Rolleiflex 6000 cameras
This is really a neat accessory for studio work when you are not able to have an assistant. I used one of these foot operated switches when I took my award winning photo series of my hands called "Opportunity". I set my camera up on a stand pointing down towards my hands and stepped on the release when I had my hands in the right position. I don't know of any other cameras that have an accessory like this and for me it was indispensable. I can imagine set ups where you needed your hands to hold objects or light modifiers in position and where this would literally give you an extra hand.
I don't really know how many of these were made, but I've got one in my personal kit and this spare one and have only seen a few others for sale in all these years.