This is a thin 6mm square wrench / key (Vierkant), that's designed to sit flush to the surface of the key “hole”.

I made it so I always have the key there, when I need to access the fuse box, as I can just put it in once and forget about it.

The square hole is 7mm² and the extended part is 1mm thick, so it's firm enough to allow for easy turning of the bolt.