Just a quick update, I haven't been doing much lately. I decided it would be easier to reframe the window from the outside so I've set up scaffolding and I'm stripping the shingles. As I noted earlier, the wood literally gets burned way by the sun, I tried to capture in this photo that in places you can literally see through the part of the shingle that is exposed to the sun:
Removing these shingles, I noticed that they're actually newer than other places where I've removed them. They're put on with machine-made nails, not hand cut, and the nails are galvanized. And there's roofing felt behind them. So they were put on some time in the 20th century probably.
I found this sticker on the back of one shingle:
Aha! Western red cedar. The shingles everywhere else are eastern white cedar. Which I've been told is more durable, and I now believe. Some googling tells me that the Stave Lake Cedar Mills opened in 1939. The label doesn't have a postal code, and Canada adopted postal codes in 1974, so that gives a window. That lines up with what I know about the history of the barn. The family that built it owned it until 1950, another family owned it until 1974, and the person that I bought it from bought in 1974. He didn't do much to it so I assume these shingles were put on some time in the 1960's or so. So 60 years give or take.