Metal adaptor to connect two small bayonet plugs together
Included is one metal bayonet plug which fits into either end of the coupler.
The plug unscrews so that wiring can be connected to it, and includes a flex grip in the cap.
As the double socket has no provision for wiring, this indicates that the plug would be used to provide power to another identical plug - an unusual arrangement in any circumstances, and certainly not designed for mains use as this would leave the end of one plug with live exposed contacts.
The spacing between the metal casing and the plug contacts also suggests this is not designed for mains voltages.
This video shows the two items in more detail, including the internals of the double socket coupler.