For some reason I have until recently been unaware of rete, a mailing list for curators, historians, students, collectors, dealers, etc, interested in the history of scientific instruments. The archives (from June 2003 onwards) are available online. The list owner (the Museum of the History of Science in Oxford) will not accept messages for commercial purposes like announcing instruments for sale, etc., but otherwise all messages for academic and museum purposes are welcome. To join, send a blank message to email@example.com.
(thanks to Gustav for the tip)