I have recently re-studied , frame by frame, the newsreel film in a version where the sound has been properly synchronised with the picture. I would say that Bix does indeed puff out his cheeks for every note that he plays. It's worth bearing in mind that he does not actually play every note that the brass section can be heard playing, and that the unsynchronised (widely available) version leads to a different interpretation of sound/vision. .
Most importantly though, I did agree with Frank , originally, that it could be a Bach that he is playing; but now I am convinced that it is a Holton. I have a 1928 Holton-Clarke (identical to the one that Bix is photographed with) and a 1930 New York Bach Stad (identical to the one in the Putnam Museum). I angled both instruments to the near exact position on the frames where you can clearly see the instrument that Bix is playing, i.e. the last few frames where he is lowering the instrument prior to sitting down. I found the following to be true from this angle:
1. The bend in the lead pipe is in the same position as the Holton, further forward than on the Bach.
2. The mouthpiece receiver is in the same position as the Holton, whereas the Bach receiver is level with the curve on the bell pipe.
I can see Bix's finger going forwards a couple of times and cannot really comment on that. It is true that the Holton did not have a finger ring, but there might be other reasons why Bix moved his finger.
I am therefore fairly certain that the instrument in the newsreel film is a Holton; and why not ? the Whiteman Band had just been sponsored by them and I can imagine the Band Manager instructing them to play Holton instruments for the film, otherwise ..... !