Through multiple pauses & back-ups, I think I got it sorted. It's not that complex, but a fair bit of careful fiddling is involved! I've seen single-curve dovetails often enough, but this double curve caused me some mental gymnastics to visualise how the second curve is created.
It looks like the pins board is prepared using a circular jig & the router, to cut the curved projection in which the pins are made. This is to the full depth of the tails board. (It could be done with hand tools, but not as quickly or easily). He probably then cut the pins, and transferred the pins pattern onto the tails board (the opposite of what I do for 'normal' d-tails, but easier in this case, perhaps). The tails are cut while the board is still square, then the double curve is done with chisel work. He is using a jig to register his chisel while cutting one profile, as you see. It seems to me that this curve will be covered by the inner edge of the pins board when assembled, so doesn't have to be pretty, just a good fit. The curve on the end grain of the tails is the visible bit, so that will need care.
Give me a day or three and I'll try a simplified version on some scrap - a picture of the pieces sliding together should tell the story.....
Chers,
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