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This short video was uploaded by the Knebworth House website a few years ago. Their description reads: "An extrordinary glimpse of the author of "Peter Pan", J.M.Barrie, playing pirates in the film "The Yellow Week At Stanway ~ a record of fair women and brainy men ~ 1923" a home movie by Nicholas Llewellyn Davies and a group of his close friends, including Antony, Viscount Knebworth."
I'm sure enough that the boy in the boat is Cynthia's son Michael, who would have been 9 in the summer of 1923. Alas Nico is no longer around to check, but he looks pretty similar to still photos if Michael at around that age.
Many thanks to Knebworth House for sharing the video ...