I have a similar letter of 1733 and there are several other from this correspondence known which are discolored probably by immersion. Apart from the great Plague of Marseille (1720/22) Europe was free of plague for at least the first three decades of the 18th century. Referring to my letter, Denis told me: "My best guess is that this was private treatment by either the forwarders or the recipients, who were receiving business mail from infected areas outside Europe."
Thank you Andre
As always your help it is very important.
The provenience of the letter it is from Archive of Angiolini and Frangini in Florence 1701-1740 sold at the auction galleries of Christie's on 30 October 1984. I believe that this is the origin of yours letter too.