Kirk, some where I have references to "Commercial Phenidone" as Phenidone-Z.. Plain Phenidone doesn't keep at all well in alkali solutions, and is partly soluble in water, where as Phenidone-Z has poor solubility unless the solution is alkaline and keeps well.
A problem is Ilford use "Phenidone" as a generic "Trade name", but Mason of Ilford discusses the problems of Phenidone then goes on to state (Photographic Processing Chemistry 966/1975) "In order to reduce this trouble, two 4-substituted derivatives were introduced commercially a few years ago "Phenidone-Z" Ilford "Dimezone" Kodak. (His editor was Levenson from Kodak)
In practice Ilford only began using "Phenidone" commercially around 1951/2 because of the problems, and Ilford's PQ developers etc keep well in concentrate form so were definitely not using plain Phenidone.
I have Ilford's 1960 trade price list in front of me and Phenidone is sold in 10g -500g quantities, but doesn't say what form, that may well ahve been a trade secret. As I said earlier the Phenidone I had adted 1962 kept exceptionally well over 40 years, and there was no discernible difference in activity when I bought fress stock 2 or 3 years ago.
It's quite possible Ron (PE) used "Plain Phenidone" at Kodak in his tests against Dimezone