This is the Nonaka Boeki Acoustic Sleeve. This part was developed as an optional part for the old LaRosa model and is compatible with specific Bach trombone models, enhancing projection and altering the sound quality. When actually using the Acoustic Sleeve, insert it into the inside of the slide receiver before inserting the slide, pinching it with your fingers and feeding it in. The Acoustic Sleeve has a slight difference in diameter between the top and bottom, so insert it with the narrower end going in first. Attaching the Acoustic Sleeve makes the sound carry further and stand out. It's perfect for playing solos in large halls. To remove it, insert your finger and hook it out. I purchased this separately before buying the old LaRosa model (originally A47BO) and haven't used it much, but it has some fading commensurate with its age. There are no problems with its use. Compatible models: Bach trombones 42B, 42T, A47MLR, 42AF, 50B, 50B0, 50A3, 50T3 *Please note that it probably won't fit instruments other than Bach, so please try it at your own risk. Also, it is currently discontinued in the catalog, and the fitting with current new models such as the 42BOF is unknown. #Trombone #Artisan #Stradivarius