Ivory bakelite plug by MK with unusual spring loaded pin sleeves. Pictures and text provided by Loz Pavier.
Underneath, each L & N pin are surrounded by circular pieces of bakelite with springs behind them. This acts as a shutter to the pin by covering the first 5mm as a plug is withdrawn.
This is an early attempt at extra safety, and the terminals are labelled 'Red, Black, Green' indicating this to be pre 1969.