Original 3.4mm case screw for use on all models of Apple 12", 15" & 17" Aluminium PowerBook G4. They are also used on the MacBook Pro range.
This screw is also used in the back left side of the 13" Unibody Macbook in the bottom case. (the other bottom case screws in the unibody macbook are different sizes, only one is this size)

These screws are used on the side casing of all models and to secure the memory cover on the 12" & 15" Powerbook models. (This screw is not used to attach the memory cover of the Macbook Pro)