Alan, Agfa stated over 100 years ago that Rodinal was an alkali salt of p-Aminophenol in neutral sulphite and water with no excess of caustic alkali (hydroxide). Calbe's R-09 still meets that criteria but in the 1930's an anti oxidant was added it was also a mild wetting agent.
Agfa may have increased the level of Sulphite with the uptake of 35mm in Europe to help produce finer ghrain but they always foremd the sulphite by adding Hydroxide to Metabisulphite. Fine grain developers became more important in terms of research in the mid to late 1920's.
Metol was probably cheaper to produce and developers could be sold in powder form, that may not be possible with p-Aminophenol which was also Patented by Agfa.
It was recommended by Agfa themselves to dilute Rodinal with Sulphite solution 5-10% (cryst, half that for anhyd) to greatly extend the keeping properties, and again first publishe over 100 years ago.