That is a 1.60 mm screwdriver blade that completely cracked in my efforts to remove the screw. It broke after having soaked the screw with Kroil for quite some time. As of now, the crown wheel is still in place.
As a rule of thumb, I was taught that when tightening screws in a watch you should gently turn the screw until it comes to a gentle stop and then turn the screw additionally the equivalent of the width of the slot. This keeps the screw from being too tight or too loose.