Indeed it seems to be a more "stabilized" version of POTA. In Anchell/Troop the addition of Pyro, HQ or Catechol are discussed as potential ways of stabilizing the Phenidone enough (by superadditivity) to make POTA easier to use. Even Metol is considered (not sure why but it seems to follow Crawley's thoughts) and also Glycin - presumably for its apparent ability to make other agents more resistant to oxidation. But the suggested formulas in FDC use small amounts of these additional agents, less sulfite and sometimes additional alkali. D-512 seems quite different in its Pyro concentration.
On the other hand the formulas in FDC are intended primarily for document/microfilms. D-512 is mentioned in Haist in the context of compensating developers for general purpose films, while POTA is discussed in the context of microfilm development.