The color match with the printing ink certainly must give one pause. What is the probability that rags stained with that ink found their way to the paper manufacturer?
Since the stamps were printed on plain paper and manually gummed by brush afterward, have you determined how to distinguish slight offsetting ("setoffs" actually), from sliding the sheets onto a stack prior to gumming, versus actual colored fiber impurities in the paper itself?
@AJ Valente: Alan, this is a real question, not a challenge. Is it likely that prior to the consolidation both Continental and American were getting security paper stock for postage stamps at the same time from one supplier, particularly when one was under contract with the Government and the other was not? What is the evidence available to us to nail that down?
essayk, Its a long shot, but not very different to a slight offset seen on some issues. The fibers could have been roughed up or even airborne. After printing they picked-up some fresh ink from the sheet directly below. The sheets were then gummed and pressed to prevent curling. In that case the individual fibers would have been pressed home, appearing a part of the paper matrix.
As to your other question, I would say Yes and No. There were in essence two separate offices. One was the production office where the stamps were manufactured. The other office was the engraving department.
The engraving office used paper from various sources. For instance, there is one reference in the Travers papers (~1872) to a thin paper obtained from the Housatonic Mill in Berkshire County MA. This one owned by the Owen Paper Company (long story there), but they were not a Government contractor so far as I know.
As for the production facility, they only got paper from the approved source (a contractor known and approved by the Government). There were times of course where special dispensation was granted, such as the case with double paper. There's another letter (Feb '78) in the Travers papers outlining a two sets of printed stock (old and new papers) sent to the Government for chemical testing of cancelling inks.