Huh, that's interesting - Preuss wrote that everything between 1721 and 1728 was lost. I'm wondering if 1726 might actually be the 1728 one. The volume of German correspondence at Trier doesn't really have anything interesting 1740-45 - not only that, it also shows how rare German letters are, especially to important people. The one to Fouque for example is a secretary one - the rest of their correspondence is in French and in another volume entirely.