My '74/'75 Devon still has a sticker with a picture of a tap on that switch. It was rewired to turn on the water pump on the sink unit as well as the internal light at the back - the 1970's striplight shown in Barneyrubble's picture. One of the PO plonkers pulled out the wire from the light at the back that linked it to the pump, so I threaded some new wiring back along behind the headlining. As the original wire was a bit weedy, I fitted a relay to the leisure battery at the back and then switched the fridge, lighting, 12V sockets etc on with the switch.
That's how mine works like a master switch, I can leave the rears switched on then operate them from the front.
