Mounted to the axle bracket is a metal disk the diameter of the brake drum (14").  The disk keeps road dirt and debris from getting into the brake hardware mounted inside the drum and provides for a cleaner look to the car.  The photograph is of the right rear wheel.

Also detailed in this photograph is the axle with the square end.  The wheel hub slides on the axle and the squared end on the axle mates with a square hole in the hub.  The back end of the wheel hub is conical and mates with the conical piece on the axle.  As the axle nut is tightened the conical wheel hub is pulled onto the conical axle section and insures a tight fit.  The circular black section between the conical axle section and the metal disk is where the wheel bearing is located.