Posidrive screws are often used in military and government electronics where damaging the screw head or liberating metal fragments from the screw head isn't acceptable. They are identified by four tick marks at a 45 degree offset to the head. Here's an example of a number six posidrive screw (magnified for visibility) from an old HP spectrum analyzer that I have.
The advantage of posidrive head and driver is that the rotational force does not "cam" the driver bit out of the fastener head like happens with a philips head. The screw head and the driver have a unique design to the flanks that makes the engagement of the driver and head positive This allows proper torque to be applied without any damage to the head and without liberating metal fragments. In military hardware, flight hardware etc. it can be inconvenient for random fragments broken off of screwdrivers or screw heads to become loose in the electronics.
Here's an article showing different fastener heads. I've never seen a posidrive screw that wasn't identified with the hatch marks, but I suppose it's possible.
These old screws are easy to remove if you don't need to save the fastener. As mentioned earlier, soak it with PB blaster. Heating the head with a torch and spraying it with PB will help it draw oil in. After that has soaked a few days, take a die grinder with an .040 cut-off wheel and slice a deep screwdriver slot in the fastener. Match the width of the slot to your falt screwdriver. Use a large square shank flat screwdrive with a wrench on the flat to apply torque. (That is why some screwdrivers have square shanks, btw) If someone had tried to remove this fastener with a #2 philips driver, you're unlikely to salvage the screw anyway. (Don't ask how I know)
