Thank you for this! I've been meaning to ask you: How many letters between Fritz and Fredersdorff are there in the database? We've gotten our hands on a couple volumes published a hundred or more years ago, and we know we're missing some of the correspondence, but we don't know how much. Of course, we also don't know how much was censored from individual letters, but we'll start with "are there entire missing letters?"

cinchona

For cahn: the bark was used to treat malaria. And possibly other things, but malaria's what I associate it with. We know Fritz had malaria in 1740--he was having a flare-up when Voltaire reports first meeting him and taking his pulse in his sickbed (god knows if this story is true, I'm so cynical about memoirs now, and never more than Voltaire with an agenda). So if Fritz is sending him cinchona and Voltaire is telling the truth at all in his memoirs, there's a haunting symmetry to their first and last meeting. (Fritz having malaria that week is canon and externally attested; Voltaire meeting him at his sickbed may be made up. But something to consider for fiction, at the very least.)

Not sure if Fredersdorf was a physically imposing person

We've always seen him described as tall. Our headcanon is that he was just short enough not to be recruited by FW as a Potsdam giant, but much taller and he would have been.

His height is a plot point in the absolutely delightful excerpt from his secret diary crackfic that cahn wrote. (cahn, ILU!) It's based largely on Wilhelmine's account of her visit to Berlin for Christmas of 1732.