Another update:
The (incredibly nice) guy who built the replica of the Seekrit Project sent me a rare, out of print book last night, and I stayed up until 2am devouring it. It contained all sorts of terribly useful photos of people, places and devices that I desperately needed for the book.
I also went to K college and made photocopies of a 150-year-old book with a very long article on the subject -- one that I'd been trying to track down for about two months, and lo, there it was literally in my backyard.
Unfortunately, what these two (really good, really thorough) items taught me were that I had several minor, but key, details wrong. Like for instance, Mr. M___ was not thin at all, but described, firsthand, variously as "stout", "florid" and "phlegmatic". Neither was he buried with his c.b. -- for it still exists, in a museum in a large city on the East Coast; and it is very unlike how I've drawn it: it doesn't fold, and it's full of holes. Mr. M___ also spoke a language (fluently, it appears) that I had him using a translator for. And neither (again firsthand) did Captain N___ ask Mr. M___ for their last fateful interaction; it was the other way 'round; it appears M___ was that desperate.
Blargh. Rewrites. Redraws. This is what I get for being so thorough and not just pulling things out of my ear. Still, far better now than after I've shown it to everyone.