They're typical of British fittings of the 1940s/1950s era.
I mentioned elsewhere the possible switch interlock to prevent a switch being turned on until a plug was inserted, but there were definitely outlets which when mated with the correct plug did the opposite function, i.e. prevented you from removing a plug with the switch on. The recesses in the earth pin of that plug provided the function.
In the "old days" here, our Regs. specified that outlets on a D.C. supply had to be controlled by an integral or adjacent switch. There was no such requirement for A.C. outlets.