Very interesting reading! Yes, I know, I'm an extreme geek!
I wonder if the original American plug was intended mainly for Line (120V) + Line (120V) + Neutral, thus making it a 240V plug?
It seems most plug designs were fixed by ca 1960.
The Schuko existed, so did the American and the South African (old British) plugs.
I think the standard for the Swiss plug is from 1959, making it somewhat newer. Of course, this could simply be an update of an older standard. The Israeli plug is also rather new.
Don't know how old the French, Italian and Danish plugs are, but hardly any younger.
