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 This acts as a shutter to the pin by covering the first 5mm as a plug is withdrawn.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.
This video shows the MK sleeved plug in detail, and compares it to an unsleeved MK plug from the same period.