Yes, that Britmac with its "rounded square" shape was less common than the plain round types.
As Alan mentioned, the usual method of installation here involved a wooden pattress block, with individual holes drilled to line up with the terminals. The block was then fixed to the wall (sometimes with just a single screw through the middle) with the wires left poking through the holes, and the switch then screwed onto the block.
I have loads of the Crabtree tumbler switches. When I was a kid these were the sort of thing that were being stripped out of older houses for rewires, and I managed to get holds of lots of them for free. They were very handy for my various electrical projects of the time, such as banks of light swiotches for model railways etc.
I'll take some pics of the Crabtree switches next time I have a photo session.
