The Denis Wick 'extending tube' mute (also known as a Harmon or wah-wah mute).
Manufacturer's description: "Poor intonation and projection have long been problems associated with extending tube mutes. Wick has eradicated these problems through a new design that creates a change in color without muffling the sound. Whether the stem is in, extended, or removed, these mutes produce a crisp, focused sound that projects easily."
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