This is a plug-in fifth rotary valve for Yamaha 321 euphonium. It should install without tools, and allows the full chromatic scale down to the fundamental to be played on this horn.
March 2019: Sadly the maker of this has ceased operations, so we have one left and that's it.
Long before they offered any compensating euphoniums, Yamaha sold a rotary valve attachment for the 321 euphonium that allowed the full chromatic scale played with five valves. This is the style of most German tubas: 5 inline valves, non-compensating. The brilliance of this valve attachment was that a player could have most of the performance of a compensating euphonium at a much lower price and much less weight. The rotor could easily removed without tools when not needed. That part has long been discontinued, but Stauffer Brass in Los Angeles has created an improved version. It has an improved mini-ball linkage, a proper .610" bore and improved airflow.
This new little part should plug in to any Yamaha 321 euphonium, and replaces the fourth valve slide.
This rotor tunes to a flat whole step. That means the low Eb is right in tune played as 4-5. You can also think of the "flat whole step" tuning as being a whole step in F, just like most tuba fifth rotors. The rotor is easily removable so you can return to a four valve instrument when you prefer less weight. The valve lets you play the entire chromatic scale down into the pedal note range, using fingering patterns that are the same as most any five valve tuba. Here are the fingerings for the notes below the bass clef with five valves:
Available in silver plate finish. Made in USA. Fits only Yamaha YEP-321.