No, not really... The one thing I know for sure is that I have that old receptacle where you push in the plug and feel it lock with a "snap". So it definitely has some spring-loaded balls or similar thing inside to keep the plug in.
The dimples theory makes the most sense, the confirmation of that would be a NEMA manufacturing spec for the recepticles that indicates where a dimple should be. But I cant seem to find any SOLID evidence.
Thats also where the inspector likes to put the red tag when he finds a non approved piece of equipment.That way you can't say you didn't see it is a $5000 fine for pluging it in.The holes and nibs just give better mecanical resistance as not all mass produced plugs are the same.Hospital grade plugs are no different than spec grade(made on the same assembly line) they are just found to be "tighter" during inspection so they get a green dot.We actualy have a gauge made by hubble to test them with cheers
[This message has been edited by frank (edited 12-04-2006).]
We had a little hardened steel pin with a ring on the end in our LoTo kitsYou stick the pin in the plug holes, run your LoTo lock hasp between the plug and the pin, between the prongs and back down through the ring on the pin. That holds the pin in and prevents using the plug. This thing was as hard as a file so you couldn't cut it with side cutters.