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 for many years from the 1950s through to the 1970s. Basically I'm referring to the one where the switch on the socket is slightly set back, with the recess being curved at the top and bottom. As a kid I remember seeing brown versions of these in my grandad;s house (built late 1950s) and ivory versions in my uncle's house (built around 1970). My primary school (built 1976) had white plasttic/chrome metal clad versions of these too.
We had our house rewired in 1981 when I was around 5 years old and I remember the very "square edged" modern looking sockets with the 'On' moulded into the switch and the textured red piece on top of the switch, visible only when the switch was in the on position. I do remember (as someone mentions above) this design being refreshed in the 1980s with the moulded 'On' being removed, the MK logo being recessed into a rectangle and the texture disappearing from the red bit at the top of the on switch. I noticed this as an extension at school in the late 1980s had MK sockets fitted and these differed to those we had at home - that is why I'm aware of the refresh.
Wundaboy suggested that the logic range was first released in 1975. I'd always guessed something more like 1977 - similar time to when the Ford Cortina suddenly went from a very curved appearance to a much squarer profile on the Mark IV launch!
I think there is an early/mid 1970s design that many people forget about, which either came inbetween the curved and first logic designs or ran in parallel with the last few years of the curved design. My grandad stayed in a nursing home that was built in 1974 (remember 25th aniversary in 1999) which had single sockets like those on the very informative Flameport website (top of page
http://www.flameport.com/electric/old_wiring_accessories/mk_socket_outlets.cs4). However the room light switches were of the very square flat rocker logic type with the chunky (wide) switch, so this design of lighswitch was around mid 1970s - introduced before the sockets changed, as wundaboy suggests from his 1972 MK catalogue.
The primary school I attended was built in 1976 and the singles sockets were the chrome/white metal clad versions of the traditional curved MK design with recessed switch, suggesting that the traditional curved design ran in parrallel to that on the Flameport site, or simply that a metal clad version of this socket was not available, so they just continued the orginal design for metal clad until the logic socktes arrived (in what I think is 1977ish). The metal clad lightswitches in the '76 shchool were gridswitch but with the earlier angled rocker, rather than flat, suggesting that gridswtich has always moved later (from dolly, to angled rocker, to flat rocker).
Any other thoughts appreciated - had to chip in as this has always been a mystery for me too.
Something which may be of interest, my 1963 MK catalogue has both the 'dolly' (described above as toggle) and 'rocker' versions of their traditional 1950s - (mid 70s??) lightswitch plate included and states that the 'rocker' (being the angled rocker not flat rocker like logic) was being introduced that year as an alternative to the dolly. I always thought that the angled rocker had replaced the dolly but much later in the 1960s. Perhaps they ran in parallel for a while? Although, thinking about it, my uncle's house (built 1970) had all dolly versions, not rocker. All a mystery.
I would love to see MK catalgoues from every year as they are of great value when trying to work out when work may have been done!
