Thanks,emeritus for the 1893 GEC catalogues.
From the GEC catalog I do see the first plug socket system. Hubbell did not know about it I suppose. He just noted that the janitor at an arcade had to remove the power wires separately from a panel and had to note the proper order before moving the machines in order to clean around them. That's when in 1904 he tried a round pin design,then the tandem blade (still used for US 240V),and finally the parallel blade design.
Grounding pins showed up in the 1920s both with US and UK plugs. I always wondered why the German Schuko plugs were not polarized,then I read somewhere both pins carried 120V to make the 240V,so there was no such thing as a neutral wire.
I can still get a BS546/5A 2 pin adaptor at US hardware stores. They call it a "shaver plug". Must be a few places that still have that kind of socket.
The same catalog does explain why Japan has a 100 V system. They decided to stay with the original 100V lamps and everything else followed.