Chances are that's true, but I'd like to see the evidence. Was there a clue found in his papers? Did his wife or agent or writer friend say so in an interview? Bill Pronzini listed the book as ghost-written by Bellem in Gun in Cheek back in 1982, but cites no source. I ain't disbelieving, just wondering out loud.
Half-Past Mortem, published in 1947, was the second book in a series (of two) featuring insurance investigator Sam Whelpton. Several online sources also list Bellem as the author of the first Whelpton book, Liability Limited, too, but I discount that as mere assumption. On the Mystery File, Steve Lewis makes the educated guess that Bellem may have been a friend of Saxon's, and stepped in after the guy the wrote the first book and died. But he admits it's only a guess.
"John, a Saxon" is a mystery, too. It's about as subtle a pen name as "Pierre A. Frank" or "Giuseppe A. Paisono." So who was he really?
I have not read (or even seen) Liability Limited, so I'm no help on that end. As for Half-Past Mortem, all I can deduce for sure is that the author was at least an acquaintance of Bellem. That evidence appears on page 163 (of 250), when Whelpton impersonates a phone company employee to find out who a number belongs to. He tells us this:
I manufactured a name and address out of thin air. "Is this Mrs. Belle M. Leslie of four-fifty-five North Raymond Street?"
Sure, that could have been Bellem playing games, but it could just as easily have been W.T. Ballard or another friend giving him a shout out. And why "Raymond Street" instead of "Robert Street"? Who's Raymond? And speaking of Ballard, it seems possible he could have been a co-author, as he was on Shady Lady and several of the Jim Anthony pulp novels in Super-Detective.
The book offers no other solid clues. The first person narration is nothing like that of Dan Turner, but that doesn't disqualify it, because Bellem wrote in many different styles. The only Turnerish word in the whole book is "yapped," and that's used only once. There are several lines reminiscent of Dan Turner's interest in women, but that sort of thing was common in detective fiction of the time:
A few minutes later Natalie came out, clad in white slacks and a rose-colored sweater. I noticed she was wearing slacks a long time after I noticed that she wore a sweater.
I glanced at Burdick, but he wasn't looking at me. His eyes were on Gloria's well-filled nylons. I had noticed her legs myself and at another time I would have been more appreciative. Right now I wasn't.
I walked out into the reception room. As I did, Dupont's buzzer rang and Doris picked up some papers, started toward his office.
She sure looked good from the rear.
As a novel, Half-Past Mortem is nothing special, but there's nothing wrong with it, either. I enjoyed reading it, and probably would have even without the Bellem connection. I give it one thumb up.