I was curious enough to get in touch with the author, Gary Pomerantz, who was kind enough to send a prompt reply:
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Hi Jonathan, Thanks for your very nice email. On the night Jack Bennett was killed, the Hofmans were interviewed by Kansas City police. Both Mayme and Charles Hofman indicated that they could not remember the distribution of cards in the so-called "Fatal Hand." Myrtle was delirious on that night, and into the wee hours, and was given a sedative. To my knowlege, she was never asked about the distribution of cards. My Best, GP
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I agree with Mr. Pomerantz that this makes it almost certain that the deal was a fabrication. What I gather, though, is that he doesn't have any first or second-hand accounts of the actual concoction, but as far as he knows it first appeared in The Bridge World so he assumes they were the ones who made it up. He is very likely to be right. If there were any bridge-playing cops on the scene (not so far-fetched in 1929) they *might* have been able to reconstruct the deal, but one would expect there to be a record of such a thing happening, and The Bridge World might have mentioned that, so I really doubt such a reconstruction happened.