Hi,
I've about half a dozen switched MK plugs; brown, cream, and a white one! I've also got an MK plug with spring loaded sleeves as mentioned recently in another thread. On most of the MK plugs I have, the locating piece that fits into the hole next to the earth pin has broken off.
I wonder if the hole was to reduce the chances of damp condensing inside the plug. Something that seems unnecessary these days as modern plugs don't have said hole.
Cheers, Pete