Yes, this is interesting. I think the issue in practical use is whether the developer lasts long enough, and whether the developing times are too long for some to be practical. But that p-aminophenol can be made to act in different ways depending on the formula (just like metol or phenidone) is good to know. I have read somewhere that using Rodinal at much colder temperatures (16 C) also gives finer grain, and the converse is of course also true. It is also super-additive with ascorbate etc. That out of all the possibilities we have only Rodinal as a widely-used developer, is then strange given how many metol and phenidone formulas there are. I can only guess that it has to do with cost and/or a lack of longevity. Do we have any indication how long the original Rodinal (not the replenished) lasted? My understanding is that it was used as a stock solution, not single shot concentrate that had to be diluted. With those sulphite levels, the sulphite would be the major contributor to cost, not the developing agent. Which might explain why it wasn't pursued commercially.