This Marelli fan O/250/P has bullet shaped head.
Many other fans have them so maybe the concept is shared between many other fans.
As you can (hopefully) see from the photos the head is attached with some "two screw system"
In the center is the smaller screw which rotates about half the turn and then stops. This fan is not rusted and all screws gave way at normal torque value. This one seems to stop solid.
The brass-looking ring around the small center located screw has a partial slot but is not budging - and I am not forcing it either because I have no understanding how this works. Does anyone here know?