hi
I discovered this type for the first time when doing the diff on my 230TE
, they are called 'spline' or 'ZXN' heads over here - pretty standard on brake calipers and such on European cars.
I've heard factories like to use them on things they don't want DIY'ers to mess about with, as we won't have the tools to undo them, and so won't be able mess with things too complicated for us....
Easy enough to find at your favourite 'oooh shiny!' shop (tool merchant)...once you know what to ask for!
cheers!