vacuum wipers again

can anybody tell me if the little black slide thing on top of the wipermotor is made of rubber or hard plastic
it is the thing that is operated by the dash knob

 

From looking at one, it looks like it must have been soft rubber when new. I say that because it needs to be a perfect seal.

 

yeah , that`s what i thought , mine is hard as a nail ,
i grinded it so it seals again , but my first idea was that it should be rubber