The plug is not a single, two-pronged plug.
They are two separate thingies, with each hole serving a different purpose.
The hole towards the front of the camera is a regular PC-flash synch socket.
This connects to the rear baffle shutter, and is of use only when a lens without shutter (microscope, micro lens, or similar device) is used, i.e. for 'open flash'.
The flash is fired when the two rear baffle doors are open, which is long before the shutter in the lens opens. So when used with a regular shuttered lens, the flash is not recorded on film when triggered through this side socket.
The limited use of the thing led to it disappearing in the early 1970s
The hole towards the rear is a socket that takes the cable hook: a tiny ?-shaped device that you hook a flash cable in when connected to the PC-synch socket on the lens.
Later lenses have PC-sockets with a friction grip, and the cable hook was no longer necessary. So the socket disappeared too, a good many years later than the body synch socket.