In the fall of 1972, I spent a whole afternoon and evening with Hoagy Carmichael, in his apartment overlooking Sunset Boulevard, at the western end of what we Angelinos call "The Strip." It was a wonderful experience - we played piano for each other, he autographed some 78s I brought over, and he even gave me a couple (one of them was "Riverboat Shuffle" by the Cotton Club Orchestra, on a "Flag" label Columbia).
Having read about the famous Bix mouthpiece, I asked Hoagy if he still had it. He brought out a tiny glass case, enshrining a Bach 10-C cornet mouthpiece. He removed it, let me examine and even blow through it, in a horn he had handy. The rim felt uncomfortably narrow - even as a fledgling cornetist I already was used to a 7-C. Later, I learned that the narrower cup sizes were good for shoring up a faulty embrochure, enabling one to hit high notes at the expense of tone quality. It all makes perfect sense if this was the actual mouthpiece Bix was using at the end of his life.
As Hoagy was a big hero of mine, I drank in every detail of this visit, and am quite sure about these facts.