Thank you for viewing. This is an acoustic balancer from Atelier Momo, now Brass Lab Momo, for large-bore trombones/euphoniums. It's a balancer, or rather a ring, that has undergone the best acoustic processing. It has also been called a mouthpiece sleeve. I don't know if it's currently sold. It's similar to Jun's Resonance Disc, etc. It's probably discontinued, but it might still be sold. If you know, please comment. Regardless of the name, it's a ring that changes the weight balance of the mouthpiece and affects the resonance. The body has been hollowed out and is not plated. You might want to have it gold-plated or silver-plated to your liking. Insert the side with the mark towards you into the shank of the mouthpiece. The sound density and high-note control are excellent. The resonance becomes unified and the sound carries further. I obtained this on this app last year. I don't usually use large-bore instruments, so I never used it in a performance. I'm offering it for sale in the hope that it can be useful to someone who needs it. There is no distortion. It fit all the large-bore mouthpieces I own.