Other Seckendorff, at a guess, might be Christoph Friedrich von Seckendorff-Adebar, but that's just because the birthdates and dying dates make it possible.

Blanning to the rescue: he names the author as Baron Christophe Louis de Seckendorff. Preuss agrees. Apparently, Blanning talks at length about Manteuffel and the Seckendorff diary (and even "Junior"!), and it all went whoosh! over my head at the time. That's why rereading him is next on my list after Lavisse.

We've come so far in so few months!