Pozidrive (not Phillips) head screw to fasten the drive flange to the Cooper S brake disc. 0.58" thread length. Used also on interal door hinges, Mk3-on. See larger photo for comparison with similar screws. Ref: SF604061, HDWS293.
Note: a set of correctly-sized Pozidrive screw drivers is handy for work on British cars of this vintage, and helps to avoid damage to the screw slot.