That style of outlet was ofen used as a DC battery charger inlet on the back of older fire trucks. There was a cord connected to a battery charger with a cord cap at the station,with the receptacle mounted on the tailboard of the pumper. When the truck left the station, the cord would pull out if the driver forgot to disconnect it. I restore old fire apparatus for a hobby and many have this receptacle. I bet this was also used for DC in bldgs. Robert
