Information about Mute model 342:

The "Dizzy Bop Mute", "Popy BOP Mute" and "Copper BOP Mute” was originally made and delivered with a roll of Felt in the hole, to hold and fix the Stem-cup. We still make the mute like this, however, for more open sound and free-blowing feel, it may be advantageously replaced with a similar roll of Rubber-Cork or Synthetic Cork material.
This will be supplied from us either with the mute or on request from you.
Don’t be afraid to experiment with this, and as a costumer, you are most welcome to contact us for any question regarding our Mutes, corks, etc.