Ah Ody, read your post and Ash is a little confused. The case they are using for the 'diversion' is the fiddler's case as he puts it.
Here is Wyatt's Post:
Locating the young gunfighter, Wyatt removed the case he’d retrieved from behind the couch from underneath his arm and tossed it to Asher.
The case behind the couch was the one Jessy had hidden and she had the one with the Violin. Matherson has his own case which is the one with the 'drugs' in it.
Just wanted to point that out in case you wanted to correct your post, or leave it that way to show that even Ash is confused by the 'shell game' with the identical cases.
Later Wyatt said, “Mr. Matherson,” Wyatt did everything he could to keep the way he felt about the man and his devices from his voice. “I don’ much care ‘bout no medical diagnosis equipment, but the case could be mighty helpful in dealin’ with these mercenaries. If’n you don’t mind, I’d like t’ borrow it. I’ll return it in as good as condition as I can.
So he switched them when you weren't looking.