HK FACTORY trombone mouthpiece ring. Hammering is applied to the surface that adheres to the mouthpiece on the back (apparently this is a "reverse hammering" specification). The plating is rhodium plating, which is a slightly special specification. Here's the explanation from the official website: While general silver accessories often use silver925, this product uses a higher purity Silver950 material and undergoes heat treatment, hammering, and special processing, designed and developed with the following four concepts: ① Tightened sound through hammering ② The resonance of solid silver (intentionally creating gaps to resonate) ③ Sound changes due to heat treatment ④ Realizing these three in a detachable form 《 R Series (Reverse Hammering) 》 ・Width: 5mm ・For those who want a more focused sound ・For those who tend to have their pitch drop in the higher register ・For those who feel the high range is too thin My personal impression is that wearing the ring makes the sound more focused and improves its projection. I've decided to sell it because I've had fewer opportunities to use it since I changed mouthpieces recently. Please check the photos for details.