This is a mouthpiece which is often recommended for use with the bass trumpet. My guess is that it works best with a piston-valved instrument. In any case, it was not a good fit for my rotary valve Bb bass trumpet. I found it was too wide, even though it is the narrowest in the range (old mouthpieces I could find for this particular instrument are narrower) and for this reasonit did't work satisfactorily for me.
None the less, it is a beautifully made, solid and heavy mouthpiece, definately of high quality, and will, I am sure, be of good use for jazz trombonists and, potentially, players on piston valve bass trumpets.