Hi David,
Regarding what I agree is a puzzling discrepancy, namely between the video on March 2nd and the tweet I cited three days later, I've DM'd Christo Grozev asking for clarity and will forward what I hear back, if anything.
Of course he wrote the following in that March 5th thread: "Ukraine had previously leaked fake FSB letters as psy-ops." So I would assume that's very likely what he was referring to in the video.
Anyway, you've read his rationale for taking it seriously, so I won't echo it here. Beyond observing that Gulag.net is an utterly credible and ethical entity, and when Grozev says he ran the text by some FSB types who believed it genuine, believe me he's well positioned to have done just that. As lead Russia investigator for Bellingcat, he has authored investigations that uncovered the identities of Alexei Navalny's putative assassins; the Russian officers who shot down Malaysia Flight 17 over Ukraine; the Skripal poisonings; etc. Check out his bio, he's got quite the record. In fact I'd go so far as to say he's a kind of searchlight illuminating things that some very powerful interests would like to keep dark, which is why I thought that thread worth passing on.
Best wishes from Ottawa,
Michael
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Michael Benson
Kinetikon Pictures