"When Bix died, Helen Weiss gave the mouthpiece of his cornet to Hoagy Carmichael. Hoagy displayed this mouthpiece on his fireplace mantel, for years, and it was a Holton.
The New Orleans Jazz Museum has a cornet donated in July 1963 by Ben Pollack ... The mouthpiece is a H. N. White Co. # 41.
The Beiderbekce heirs are left with two moutpieces. One is a Bach # 7A, and the oher a H. N. White Co. # 6."
One source tells that Hoagy kept Bix's mouthpiece in his pocket. See
"Carmichael idolized Bix's innate genius; their friendship would influence Carmichael's songwriting for the rest of his life. He would later name his oldest son Hoagy Bix Carmichael after his late tormented friend and even carried Bix's mouthpiece in his pocket until he died."
The most reliable version is the one about the mantelpiece. From Sudhalter's "Stardust Melody," "The Beiderbecke cornet mouthpiece resting in a glass case on Carmichael's California mantel is more than a keepsake. Talisman and totem, it preserves a link with the living Bix who still inhabits his friend's everyday."
I understand that Bix's Holton mouthpiece is now in possession of Hoagy Bix Carmichael, Hoagy's older son.
To a degree, Bix's magical tone was helped along by the equipment he used.
Much of his playing was on Conn Victor cornets. All the old Conns--reeds and
brass--were a little heavier and had rich tones, and the bore size of the
Victor Cornets was extraordinarily large. The standard bore size for cornets
and trumpets is 460 thousandths of an inch. The standard for large bore
horns is 468 thousandths of an inch. The Conn Victor bore was 484
thousandths of an inch. Most modern trumpet players have never seen, or even
heard of, a horn larger than 470. And, Bix mostly used an old-fashioned
cornet mouthpiece that was deep and open, compared to the modern ones. This
large bore and open mouthpiece would cause any cornetist to sacrifice some
endurance and some upper register accessibility. The payback is an enriched
tone.