The metal knob actually is PE pin for the AC plug, which is used in some parts of the world with presumably stricter low voltage safety standards which would no

The metal knob actually is PE pin for the AC plug, which is used in some parts of the world with presumably stricter low voltage safety standards which would not allow for the chassis of macbook to float at what is essentially half of the mains voltage (this is why you get slight tingling sensation if you run back of your hand over macbook that is plugged into charger with two-prong plug).