Longer Pozidrive screw to secure the rear drum with built-in spacer. 1.21" thread length. Also used to attach Mk3-on door locks. See larger photo for comparison with similar screws.
Remember, many items on British cars of this vintage used Pozidrive screws; regular Phillips head screw drivers will damage Pozidrive screws. Buy a selection of the correct-sized Pozidrive screwdrivers for the best results!