Chosen Solution
Where can I find a proper bit for this kind of screw?
These screws on the bottom of the battery case Update (11/15/2017) This power cord is produced by a Czech company Emos model HE1313 (E1313).
Here’s the bit you’ll need: Tri-Groove-Nut-Driver 1/4" socket
This type of screw seems to be common in power strips in (some parts of) Europe. Its head is only about 5mm in diameter at its thickest, and they tend to be seated quite deeply within the power strips, making extraction difficult. It also seems to be a genuine “security” screw because it really is difficult to find a driver for it. The closest match in dimensions seems to be the “#6” tri-groove socket that’s sold in the US, but on the pictures it appears too thick to fit the deep screw holes in these power strips. I’d managed to drive it with snake-eyes but it wasn’t efficient and it definitely won’t work for fastening it back into place.
