Again, go to that last chorus, listen to the juncture of the end of the bridge and the beginning of the last eight. The soloist is blowing, hanging onto his last note, and as far as I can tell, still going and almost completely out of breath when Secrest enters at full volume, with a crisp attack, backed by a whole fresh bellyful of air. (This all is clearly audible in Hans' new transfer). Therefore, in the absence of multi-tracking, we must be listening to two different players, the first of whom is not Andy Secrest, slightly overlapping.
Something like this happens at the end of the Trumbauer "Mississipi Mud," when Bix comes to the end of his amazing solo. There is an eight-bar interlude on trumpet/cornet, leading directly to Bix's final lead to the end of the record. It SOUNDS like it's Bix playing the whole time - solo, interlude, out-chorus. But there's a slight overlap of horns between the end of the interlude and the start of the out-chorus. Of course, that's Charlie Margulies in the interlude, sounding uncannily like Bix in an almost perfect segue!
Now, Margulies is supposed also to be on the Mason-Dixon sides, and therefore, just on the basis of his presence, could have played that bridge. But I think not. That bridge is improvised, created as it went, and Charlie was always a reader. I still say it sounds like a very careful and circumspect Bix, and since Bix was in the same studio the next day, that's all the more reason to think so.
I suggest that Bix had ended his self-imposed recording hiatus, deciding to try again, at least at Columbia, by May 15th.