Double socket outlets made by MK.
Single oval MK logo at the centre of the plate.
The outlets are not vertically centred on the plate - they are positioned towards the top, leaving a much larger space under the live/neutral holes. This would have allowed this outlet to be fixed very close to the floor, as there is more space for the flex to bend underneath the plug.
Two fixing holes, same spacing as modern sockets. The outlets are vertically centred on the plate.
This example has signs of overheating on the left outlet - the hole for the live plug pin is discoloured. This was probably due to a plug with a loose connection, causing the plug pin to heat up when in use.
DIY decorators have also been at this, which is why there is emulsion paint all over it.
Unlike the other examples, this has four fixing holes, two on each of the longer edges. Boxes of the same age as this outlet typically had 6 fixings - two on each longer edge, and one on each shorter edge. Modern sockets with two fixing holes will therefore fit onto old boxes, but old sockets like will not fit onto new boxes, as they only have the two side fixings.
This example was removed from a kitchen, which is why the switches are caked in ancient grease.