From Berg Larsen:

Black Ebonite

This is where it all started! Since 1945 Berg Larsen's mouthpieces were manufactured from black ebonite (hard rubber) which is the traditional material for mouthpieces. Ebonite has certain absorbent qualities which give a more resonant tone.

Stainless Steel

Stainless steel mouthpieces were developed for increased life and for their different tonal quality.
The reflective nature of stainless steel results in a brighter, more edgy sound.
The insert is made of high grade black ebonite.