Still working on the puzzle. The published edition of the selected letters of Famous Author has one letter to MPF, but in keeping with some principle entirely unrelated to my convenience or that of other researchers, declines to give the collection name, let alone box or folder numbers, for that letter. The notes indicate only that the letter is in Well-Known Research Library.
Well-Known Research Library's finding aid for the collection on Famous Author has no record of the letter, or indeed of any letters or information relating to MPF under any of his names. WKRL has nothing on MPF in any collection, finding aid, or reference in the catalogue.
The letter's there, along with (probably) others to MPF, but is it really there if it can't be found? If a tree falls in the forest, and no one sees it fall--oh, never mind.
I guess it's time to write to WKRL, and I'm dreading it a little because the research librarians I've written to or had contact with in the past have primarily been of two types: fantastically helpful or eager to snub researchers. Most have been the former, but as someone wrote recently in a blog post about feeling as though she's finally arrived because famously snooty librarian deigned to answer a question for her, that's not a given.