Oh, the other weird thing about this is that even though the "external" catalog information on makingscience.royalsociety.org and the "internal" catalog information penned on page 2 of the letter both describe it as a letter written to Leibniz, and sent to Leibniz, the text of the letter itself refers to Leibniz only in third person, and refers to some other individual in the second person. But it does make (oblique) requests of Leibniz, e.g.
> I question not but Mr Leibnitz may have many of those specimens by him and therefore I doe heartily wish you could prevail wh him to Communicate some of those which would be a means to persuade severall yet incredulous of the possibility of such a Science.
I suppose it could have been written to Leibniz's personal secretary, or some such. If it weren't for all the catalog data I'd assume it was written to some close colleague of Leibniz instead. Anybody want to track down a plausible explanation/mechanism here?