After several rounds of gassing, and the specific stains were not reducing. It was time to take a closer look under magnification and see what is going on.
Under closer examination, I determined, I did not know specifically, but leaned towards the possibility that it was rust. Rust, like carbon is not going to be "bleached" away. Sometimes you have to physically remove them. The tiny small rust particulates coagulated at certain ends of wave effects and pressure points. I have determined that the two square stains at the top of the image was not from tape, rather, clamps. As seen above.
I determined it was best for the piece to go ahead a physically remove the rust (iron deposit as much as possible to prevent the stain from reoccurring in the future. I would have not done this if the stain went through he paper, but in this instance, concerning all stains, it was at the surface and a light abrading would remove 90 - 95 percent of the stained paper.
The paper effected by the contaminant had lost all sizing and fiber strength. With a very light oscillation of the bristles of a fine pit brush, the rust stained pulp beaded away-nicely. Leaving very little visible alteration of the paper (other that color and some minor inking loss).
Now the dark stains are removed, there is one round left for tone reduction needed. Before the restoration of the paper begins.
More coming tomorrow.