Yep Sparkwright - kind of a televsion screen shape but each edge with a minor arc facing into the centre - totally different from the socket plates. I have to agree about the second "angled rocker" version working upside down, certainly in the way it looks when in the respective on/off...
Hi - newbie here. I've read this topic with interest as I'd always wondered the very same questions myself - more in relation to when the curved plates were phased out.
I have a 1963 MK catalague which shows the classic single socket with curved edges and a recessed switch that was available...