A most unusual item with round and square pins at various angles. Pictures provided by Roy Rust.
The lid is secured with two screws at diagonally opposing corners. The other two screws are for the flex grip.
Plug pins are solid brass, unsleeved. There is a large rubber strain relief grommet where the flex enters the plug.
The inside of the lid is marked PATENT AND REG'D DESIGN, P5158
The lid has threaded metal inserts for the screws which secure it to the base.
Note the wording on the plug - 'BSS 1363 WHERE APPLICABLE' - the modified pins clearly do not comply with that standard, although the overall shape, pin spacing and earth pin do.
13A rating, British Made.
The flex connected contains red/black/green coloured wires as used prior to 1970, and is connected in the usual BS1363 arrangement with line (red) on the right, neutral (black) on the left, earth (green) at the top.
The presence of two fuses suggests this was not used on a 240V supply - a more likely arrangement would have been two supplies referenced to earth, for example 55V on each pin with 110V between the pins. This voltage is still used on UK construction sites today, although the exact use for this plug is unknown.