However, #4 looks something similar to what I noticed in the hallway of a hospital (here in the US) recently.
YES!!! I've been meaning to ask this question also. We have similar them on the pillars of subway train statinons in New York City. Some have been replaced with twist-lock recepts for 240 volts at 30 amps (I believe). Others with standard NEMA 5-20 sockets (for 110 volts).
They also used to be in Elmhurst Hospital in New York when I was a kid (before vast renovations).
Apparently the ones in the hospital had standard 110-volt current across them because I routinely saw wall-mounted fans plugged into them -- somehow the flat pins of the plugs did make contact with the sleeves inside the receptacles. The socket had a red box painted around it.
However the dimensions of the 15-amp British socket is MUCH bigger. The three-round-hole sockets that I've seen are closer to the smaller 5-amp British socket.
I'm wondering if these are actually those pin-and-sleeve sockets used for high amperage stuff...except those have a locking collar. The ones I saw didn't have that....they seemed like standard receptacles slightly recessed in the hole in the plate.
I remember once in the subway a crew was using some piece of machinery. It was plugged into a short extension cord that had a contemporary type female cordcap on one end and into the wall recept was stuck this HUGE brass plug.
I think it even had a threaded collar for locking the plug into place (the outlet has a collar for a protective screw cap that flips down after you've finished).