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 a discontinued item, 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 mouthpiece shank. The sound density and high-note control are superb. The resonance becomes unified and the sound carries further. I obtained this through this app last year. I don't usually use large-bore instruments, so I haven't used it even once 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.