Limited Edition TMS Kobo Original Sound Improvement Goods Rotary Spacer-kun (Copper Hammering Model) ① These are sound improvement goods to be inserted between the rotary caps of a horn. They change the ease of sound production and resonance. They have the rich resonance of copper and the good start-up due to hammering. Can be used with Yamaha, Alexander, and Holton. I think it can also be used for tuba and trombone. Outer diameter 21.5, thickness 1.5, hole diameter 10.5 Made of copper