Great story, Brad. The first question that springs to mind is: was the 10-C an "old fashioned" V-shaped cup or a "modern" C-shaped one? Lion (p.296) mentions that Bach introduced C-shaped cups for cornet mpcs towards the end of the 1920s.
Rather ominously, this is what the Bach mouthpiece manual these days has to say about the 10C: "Cup Depth: medium - Cup diameter: 16.00 mm - Rim shape: fairly wide with rather broad, flat surface. Same rim as No. 10, but with a medium C cup. Description: especially designed for high notes and sparkling brilliancy".
And although the shop cards themselves carry no information on mouthpieces, they are a reminder of the fact that of course descriptions and shapes may be expected to have changed slightly from the late 1920s onwards.
Assuming that Bix chose this mouthpiece with due consideration, 10-C is a big leap backward from a Conn Victor with matching mpc, or even a Bach 7-A. A "fairly wide" rim with a "broad, flat surface" would of course be very accommodating for a player whose chops are shot. It would have played murder on his tone though, poor guy.
Also, I'm still wondering about that Holton #27 mpc. Was it extra deep? V or C shape? Was it a Heim?