Older type MK plug with unsleeved brass pins.
The plug is made from heavy duty moulded plastic, with the sides having a raised area to provide a good grip when removing the plug from a socket.
The base has a wider flange, to keep fingers away from the pins
The hole at the top above the earth pin was provided for inspection purposes, so that the connection could be seen without opening the plug. Another, probably less accurate explanation is that some older radios had a separate wire for the earth, and this could have been connected via this hole so that is was kept well away from the live pins.
The three pins are made from solid brass, and unlike modern plugs are unsleeved - the whole length is bare metal. This is not a design defect - at the time, sleeved pins were not required or even thought of.
The larger screw near the centre is to secure the cover. The two smaller screws are for the flex grip.
The internal components are also made from brass. The pins are secured into the plastic base with a small round fastener. Unlike most other plugs, the pins do not fall out when the cover is removed. The cover fixing screw also has a small fibre washer to prevent it falling out.
The flex grip is a small flat bar, made from a compressed fibre material. The fuse holder has tin plated clips to reduce corrosion.
The inside has wording to assist with wiring the plug. L N and E markings indicate the function of each pin, and the appropriate wire colour is also marked - green, red and black.
The fuse shown here is a 2A type. More usually, the fuse would have bneen a 3A, 5A or 13A. All ratings of fuse are the same physical size, and therefore the person fitting the plug would select and fit the appropriate type.
Wires are secured under screw clamps, with the wire wrapped clockwise around the terminal post. This is a far more secure method of fixing than the alternative of a small screw directly contacting the wire and probably cutting into it.
Note the clamps are made in 2 parts, the lower part being separate so that the upper section can be tightened without twisting the wire underneath.