This is a trombone mouthpiece from willie's, which once brought about a trend of Japan-made mouthpieces. Model: U3 Steiner (Peter Steiner's signature model) Size: 5S (Rim inner diameter 25.51mm, Throat diameter 7.2mm) The U3 series is characterized by a feeling of air intake that exceeds the actual size of the mouthpiece, and this product also offers a blowing sensation that is distinct from general 6 1/2 or 5G size mouthpieces. While it improves the freedom of performance, it also requires a significant amount of breath and delicate control, so it could be said to be a double-edged sword. Even so, it worked well with my Kuhn & Hoyer trombone, and the tone was very attractive, so I used it as my main mouthpiece for about two years. After that, I had fewer opportunities to play tenor trombone, so I put it away in storage, but I am selling it to organize my belongings. The outer box says GP (Gold Plate), but please note that I had the manufacturer change it to pink gold later. Mouthpieces have also become very expensive recently, so if you are looking for an affordable, general-purpose size mouthpiece, why not take this opportunity? Thank you.