Are you sure that A&T are talking about the powdered form? My understanding is that it deteriorates fairly rapidly in water solutions, but not in powdered form. Dimezone S (a phenidone variant) is said to last longer in solution than phenidone, which is why it's used in most (maybe all) commercial developers that are commonly said to use phenidone (such as XTOL). To the best of my knowledge, though, powdered Dimezone S doesn't last longer than powdered phenidone. I've got some Dimezone S that's about three years old, and it seems to be working as well now as when I got it.
I thought that's what I thought was indicated in their book. However, it was for stock solutions and I'm might be mistaken that it also applies to powder shelf life. Phenidone my not keep well in stock solutions but, obviously, it does keep well in powder form. Let me double check the book to see if I just misremembered what was stated.
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