His Excellency Mr.... wrote Letter L-278 of sometime before November 1695 on the day he left Düsseldorf, a lost letter

This letter is known only by reference in another letter.

Leeuwenhoek begins Letter L-279 to Johann Wilhelm, who lived in Düsseldorf, with a reference to someone they know in common, but without context or details. It is possible that “His Excellency Mr....” is Luca Giamberti. However, it was not until over a year later that the exchange of letters began between L. and Giamberti and that L. began to make references to Giamberti in his correspondence with Antonio Magliabechi and Johan Arnoldi. Another possibility is Francesco Frosini (1654-1733), an Italian Catholic archbishop. See Letter L-277 ot 31 October 1695.