> Sulfite was also used as a preservative.
It is Red-sensitizer too. Again Burt was the inventor of this role of sulfite (B.H.Carroll, D.Hubbard: The Photographic Emulsion: Sensitization by Sodium Sulphite, 1933).

Ron, 3 hours ago I synthesized new Ag-excess emulsion and poured it on glass plates. This time I washed it by acid water (pH = 2,1) and tomorrow shall test it in shooting and developing.

To Hexavalent:
In this case "Red-sensitization" is not spectral sensitization at all but shortening of "Reduction Sensitization" (excise me for unhappy contraction). I mean the chemical sensitization ("after-ripening" in term of english reseachers).
Reduction Sensitization was the main companion of Sulfur Sensitization before 1930-s (when Koslovsky from Agfa discovered Gold Sensitization) - that was the time of active gelatins.