That pic reminds me of the old Reyrolle 3-phase connector we had here, before they were banned in 2000.
As djk mentioned, the metal outer on the plugs here used a scraping Earth contact, if you were lucky puuling the plug out you would get a tingle in your hand.
If your hand happened to be damp, look out!.
These connectors were used for welders and Portable Refrigeration Units (on the back of a truck).
Here's an example of the socket in the bottom right hand corner of this pic:

[Linked Image]