This is a practice mute for euphonium made by Yamaha in Japan. The mute has a pick-up microphone inside, which plugs to a small box that adds amplification and reverb. Into this plugs your earphones, allowing you to play with little sound, but you can hear yourself just fine and you won't need to force the tone nor should you tire as easily.
This is the first generation of the Yamaha mute. There is a new one that can be disassembled to fit inside your instrument. This one is less hassle to set up but it doesn't offer compact travel.
This particular Yamaha mute is lightly used by a retired player. It includes all parts, which look to be in fresh and new-ish condition. Includes mute, cord, amp box, earphones, carrying bag.