I have a box of salvaged three phase sockets. They're pretty old and of an all-metal construction. I'd like to mount a pair on my phase convertor (when I've built it) but I don't have any plugs to match the sockets. They have 3 phases, a neutral and an earth, but I don't think they're CEE sockets. As far as I know the 3P+N+E CEE sockets have the pins arranged in a pentagon, whereas these have four pins in a square and the earth in the centre. There's an emblem made from an interleaved "M" and "L" on the front. Here's a picture:Any idea what they're called? That way I might stand a better chance of finding some matching plugs...