Cinelli M71 pedals are (one of the) first clipless pedals on the market. There have been single sets and prototypes, but Cinelli was one of the first companies with mass production. This sample seems to be of the 4th generation M71.
At the backside of the pedal, there is a lever, that has to be operated to lock or to release the shoe.
This asks the rider to do more or less the same movement as to tighten or release toe straps with conventional pedals.
Same as the later Adidas System 3, the shoe cleat slides into the sides of the pedal platform. A pin pops up when the lever is pulled outwards and the shoe is locked in place. When the lever is slided inwards again, the pin moves down again and the shoe with cleat can be slided out from the pedal.