Are you saying that the replacement bumper to fit the extension (the one that has the screw hole in the end) is made out of plastic, which differs from your cue's original rubber bumper?
Please clarify.
I'd check mine right now, but I don't have it with me. What's really cool is that my extension is the one Mika gave me, which he won back to back US Opens and a World title with.
(for some reason it's magical powers don't seem to be working for me)
The Mezz cues come with a regular bumper and when you buy the extension package it comes with a replacement bumper that allows you to screw in the extension. It is still rubber, not hard plastic.