A supposedly unbreakable plug made from black rubber.
The rubber has substantial cracking around the edges due to age, and the remains of an old label can be seen on the lid.
The base containing the terminals is made from a harder material compared to the lid, and a few pieces have broken off near the neutral terminal (left side in this picture).
The lid is of the style where the flex has to be passed through the hold before wiring the plug. Someone has obviously forgotten this and therefore sliced through the lid in order to fit the lid over the flex.
Other text: BS1363A, MADE IN ENGLAND, 13 AMP, 250V.
Note that despite the poor condition of the rubber and plastic parts, the solid brass terminals and pins are still in excellent condition with very little wear.