It's interesting that sockets had safety shutters in them back then.
It's my guess that the shutters operated the interlock on the switch.
Here's an HPM horizontal double socket outlet with extra switch:
Popular here in kitchens where the switch is often used to power a socket outlet supplying either a dishwasher or waste disposal unit.
They are also used in bathrooms for shaver sockets and the switch used for vanity unit lighting.