Here is the working solution of H&W Control (microfilm developer):
Sodium Sulfite......................4.28g
Hydroquinone.......................0.076g
Phenidone............................0.52g
Sodium Carbonate.................2.19g
Water to...............................1L

So D-512 does have a much higher concentration of the secondary developer that regenerates Phenidone.
Perhaps this is to remove more of the oxidized phenidone that would otherwise give a flat image with ordinary film:
http://www.jackspcs.com/wondphen.htm