Hi,
Anyone remember this? It's an "Easi Fitt" plug made by Fitall of Larne, Northern Ireland. Pulling & twisting the earth pin releases a 'cartridge' that can be wired without a screwdriver. It's then pushed up into the body of the plug & locked by repositioning the earth pin. There was nothing stopping it being plugged in without the insert thus exposing live parts.
I'm not really sure what the idea was behind it. Service engineers, perhaps?
Cheers, Pete