I have perfomed densitometric testing on both formulations and have microscopically (optical) inspected the negatives - film from the same emulsion numbers, exposed the same and developed in both developers.

The Hydroquinone version of Pyrocat-HD does provide proportional staining and tanning (as it should with no or low sulfite). The stain image is a slightly different color (from the Pyrocatechol version) but otherwise is very similar.

To quote Patrick Gainer: "Hydroquinone is 1,4-dihydroxybenzene. Pyrocatechol is 1,2-dihydroxybenzene. According to theory, any developer with two or more active hydroxyl groups will be an active tanning and staining developer."

Gainer cites "The Theory of the Photographic Process": Chapter 13: "The Developing Agents and Their Reactions" by W.H. Lee.

See "The Role of Antifreeze in Photographic Science" by Patrick A. Gainer in Volume 25, No. 2 of Photo Techniques, March/April 2004.