Now looking at different Phenidone versions (for non chemists like me):
- Phenidone's developing properties were discovered by John David Kendall and patented as GB542502. You can find the GB patent here, but since the same thing was also patented in the US, it's easier to read the equivalent (but later) US patent US2289367.
- If you look at the structure of Phenidone and what was patented by Kendall, you see that he covered all kinds of substitutions, but only one substitution in the 4 position (the N which attaches to the Phenyl group is position 1, the N right next to it is position 2 and so on), so Kodak used Phenidone variants with 2 substitutions on position 4, most notably Dimezone S, which is 4-Hydroxymethyl-4-Methyl-Phenidone.
- Substitutions in the 4 location seem to improve something so Ilford went ahead and used
, which is 4-Methyl-Phenidone. Most of Ilford's MSDSs don't list the primary dev agent, but the one for Ilfotec HC Developer does, and sure enough it uses Phenidone B.
- Kendall's patents have expired decades ago, so would have any Kodak patents covering Dimezone S. If Dimezone S was profoundly better than Phenidone B, Ilford would have likely switched by now. Same thing goes for Kodak if Phenidone B had some profound advantages over Dimezone S.
- If you order "Phenidone" from Photo Formulary, you apparently get the original version of Phenidone without substitutions in the 4 position, also named Phenidone A. From what I read here both Ian and PE agree that Phenidone A is not long term stable. Formulary's web page doesn't offer Phenidone B, but they do offer Dimezone S. Artcraft only carries Phenidone A. Both Ian and PE would likely agree that Dimezone S is the better choice than Phenidone A if long term stability is needed.
What does this mean for us? If you want to mix a long shelf life developer yourself, get Phenidone B or the more easy to get Dimezone S. For all other applications, Phenidone A is much cheaper, works in lower concentration and is easier to obtain.
And one more thing about seasoning and consistency: since Phenidone and friends don't respond to Bromide as much as Metol, seasoning effects should be much less than with older print devs. Note that, for example, Xtol can be replenished with Xtol.