"From the northwest angle of Nova Scotia, to wit, that angle which is formed by a line drawn due north from the source of the St Croix River to the highlands, along the said highlands which divide those rivers that empty themselves into the St Lawrence, and those which fall into the Atlantic Ocean, to the northwestern most head of the Connecticut River…"
When I read this paragraph from the 1783 Treaty of Paris, it was blatantly obvious to me the green border on this map is the one that was agreed on, at least in writing.
However, you could certainly argue that the intent was the pink one. The pink one would have made the most sense on the ground (and would today help us Maritimers avoid an aggravating detour further north when traveling west for work). The Americans could hardly lay claim to the land north of the pink border, and the British would have had no logical reason to give it the Americans. The only human inhabitants of the area were French (aka Acadians) and Aboriginals (aka Indians).
Thus the compromise, I guess, between what must have been the intent and what was actually put in writing.
Considering that this paragraph almost lead to war, it might be a good idea, in future, to include a map in future treaties, eh!