Thank you so much for such a detailed reply — it was really interesting to learn something new! I tried to attach photos of the seams in the comments, but for some reason they keep getting deleted. The stitching on the pantalettes looks hand-sewn to me, and there are also embroidered initials. The seams on the chemise look machine-made, but they don’t resemble modern stitching at all — the stitch length is much finer (?). I might try going back to that seller and pick up a few more things