These are very interesting conclusions, and could be historically significant, as you said.

But I have a question. Your conclusion on its age seems to revolve around the statement that rurally hand-woven joined plaids with a herringboned selvedge ceased to be woven by around 1770. Given the fact that these were rurally woven (and thus, I assume, not done by well-documented mills), how sure are you that this older style of weaving ceased entirely by that date? Is it possible that one or two small crofters carried on this style of hand-weaving later than everyone else? I could see it being possible that a mother taught her daughter the 'old way' and the daughter made a plaid many years later, using the new MacIntyre sett that she saw somewhere.

Without conclusive proof that no one hand-wove in this style after circa 1770 (which would be nigh impossible to conclusively prove), I would think that the age of this piece remains very much in question.