Bix's Bach 7A mouthpiece would have had a standard cup diameter "7" and a deep cup depth "A". Bach used the letters A - F to designate cup depth (deepest to shallowest) and made most of his mouthpieces with diameters between "1" (largest) and "12" (smallest). The standard mouthpiece that comes with every Bach cornet or trumpet is the 7C. So this is consistent with what Bix probably would have preferred -- a standard cup diameter combined with a deep cup to produce a warm, rich sound. However, deep cup mouthpieces allow more of the lip to protrude into the mouthpiece, so the lips tire out quicker in a deep mouthpiece. Perhaps using a deeper mouthpiece made the embouchure problems Bix experienced after his health declined into more of a problem. Intermittent playing is the biggest killer for trumpeters. By the way, I play a Bach 3C trumpet and cornet mouthpiece.