It took a long time to warm up to Apple, but their peculiar way of doing things ended up as actually useful to me in practise. Sony and Korg are both on the fecal list thanks to unnecessarily special power cords on devices I own; I won't be buying any more of their products simply because making something very slightly different from a readily available stock item in order to force customers to buy otherwise low-cost spares ONLY from them is just unacceptable "dickgineering" behaviour. Compaq long ago won a Boot To The Head nomination thanks to a few of their unique (form factor, pinout) PC expansion cards. You can, no doubt, think of your own examples.
The other day I tentatively added my new/old Engravograph machine to this personal list of infamy as I found a "security" set-screw at a (what appeared to me to be) no-brainer adjustment point. Unbelievably I do not actually own the matching tool to loosen this fastener; it's not Phillips, nor Torx (not even pinned), nor Allen, nor square. I needed a magnifier and the Internet to determine the true ID, then some patience to form the right cross-section for a temporary tool.
Turned out it is Bristol Wrench spline, 4-flute. Indications are that this is somewhat of a standard application within the print and engraving industries. It was recessed in the part so slotting it was not possible. Once I had filed up a workable tip on a tool-shaped object, I considered removing it in favour of an Allen version, but I went looking a bit further.
The Maker's Bill of Rights essentially states - no special fasteners without a darn good reason - and I have to agree.
Reading up on the subject throws some light on why this set screw might different. Very small cross-section Allen sockets tend to cam out and round off if you use them a lot and this IS an adjustment screw that would (in production use) get loosened and tightened fairly frequently. It needs to be really tight on the cutter shaft, which again increases the risk of cam out in the smaller diameters.