A plastic plug which has no screws and can be wired without using a screwdriver.
It can be opened without any tools - a small coin can be used to rotate the red locking piece.
This example contains a 13 amp fuse, but it can easily be changed for other values such as 3A or 5A.
'World Pat Pend' also appears, although no patent number so it's unclear whether any patent was actually granted for this design.
The lower edge of the plug contains a metal blade to strip the insulation from the wires to the correct length - visible on the lower edge to the left of the flex grip hole.
The flex grip can be seen here at the lower edge, a threaded plastic piece which is tightened by hand.
When the lid is fitted, it prevents the flex grip rotating to ensure it doesn't loosen.
This video shows the plug in more detail, including the screwless wire terminals and the Marbo 13 amp fuse.