As you probablly know british BS1363 plugs have a fuse, normally one of the fuse clips is on top of the live pin, while the other is attatched to the live terminal.
On most plugs the fuse clips are made out of peices of springy sheet metal rivited to the pin and the terminal. In this case though it seems the manufacturer has machined the fuse clip and the pin out of a single peice of metal.
But the fuse-holder needs to be springy enough to allow the fuse to be inserted and grip the fuse once it is. Presumablly they couldn't acheive the right level of springyness by machining the top alone, so they cut a slot which acts as a lever and allows the fuse to be inserted without requiring excessive force and without bending the metal beyond it's elastic limit.
The other fuse clip seems to be of a more conventional design.
This plug is clearly old, pin insulation was introduced to the BS1363 standard in the 1984 revision, though i'm not sure when it became mandatory for plugs to follow the new revision. I suspect it would be difficult to implement pin insulation on a plug with such a slot.