Purpose of the rubber and piece of metal in a water engine pump

[JohnD]

Why are you asking?

 

[robinbanks]

the new don't have the rubber or the special piece

Probably because they realised when it becomes old and perished and detaches it whizzes round in the belt, jumping the teeth and wrecking the engine

 

[aprengo]

The rubber acts as a plug for the weep hole in the base of the pole of the water pump.
Weep hole.
So is for eventual leakage of the water pump.