Did not get a reply from Mr. Vaillancourt, so I had to wait until I had a copy of the book: The Maine Woods by Henry D. Thoreau. It's not a reinforcing thwart used in birchbark canoes. It's really a carry-board/back-rest. It does not seem very comfortable and I would not like to be tied up this way to a 65 lbs w/c canoe on a difficult carry...
So, here's the description from page 255. Thoreau's guide is a Penobscot (Abenaki ?): Joe Polis. The trip occured in 1857. Pictures are mandatory if you make a carry board and try it out!
"He prepared his canoe for carrying in this wise. He took a cedar shingle or splint eighteen inches long and four or five wide, rounded at one end, that the corners might not be in the way, and tied it with cedar bark by two holes made midway, near the edge on each side, to the middle cross-bar of the canoe. When the canoe was lifted upon his head bottom up, this shingle, with its rounded end uppermost, distributed the weight over his shoulders and head, while a band of cedar-bark, tied to the cross-bar on each side of the shingle, passed round his breast, and another longer one, outside of the last, round his forehead; also a hand on each side-rail served to steer the canoe and keep it from rocking. He thus carried it with his shoulders, head, breast, forehead, and both hands, as if the upper part of his body were all one hand to clasp and hold it. If you know of a better way, I should like to hear of it. ... The shingle remained tied to the cross-bar throughout the voyage, was always ready for the carries, and also served to protect the back of one passenger."
Best,
Louis Michaud