The passage transcribed and translated above appears in a draft letter from Count Carlo di Firmian, Habsburg minister plenipotentiary and de facto ruler of the Duchy of Milan, to Count Lorenzo Salazar, direttore of the Regio Ducal Teatro. who was in charge of local arrangements for the wedding of Archduke Ferdinand and Princess Maria Beatrice d’Este that coming autumn. (On the wedding, see especially our entry for 17 Oct 1771; on Firmian, see our entry for 4 Apr 1770.) In a letter dated 9 Feb 1771 to Count Johann Wenzel Sporck in Vienna, Firmian had written that Empress Maria Theresia had left to him the choice of composer and poet for a new serenata to be commissioned for the wedding festivities, and Firmian suggested Mozart (see our entry for 9 Feb 1771). His initial suggestion for the author was Giovanni Ambrogio Migliavacca. In his response to Firmian dated 21 Feb 1771, Sporck reported that he had informed the empress of Firmian’s recommendations, and she had replied that she trusted Firmian’s judgment (the relevant passage of Sporck’s letter is cited in our entry for 9 Feb 1771). By the time of Firmian’s letter to Salazar on 7 Mar, Migliavacca seems to have been out of the running: Firmian now wrote that he would let Salazar know later his choice of poet. Just two days later, Firmian wrote to Sporck informing him that he had chosen Giuseppe Parini (see our entry for 9 Mar 1771). In the passage transcribed above, Firmian instructs Salazar to have a letter written to Mozart in Verona informing him of the commission. This was the commission that became Ascanio in Alba, first performed on 17 Oct 1771 in Milan during the festivities for the imperial wedding.

Wolfgang and Leopold were still in Venice on the date of Firmian’s letter to Salazar. They left Venice on 12 Mar 1771, arriving in Padua on 13 Mar, Vicenza on 14 Mar, and Verona on 17 Mar, where Salazar’s letter would have been waiting for them. Leopold alludes to the letter the following day, in a letter to his wife:

Gestern habe Briefe aus Mayland erhalten, der mir ein Schreiben v Wienn ankündigte, so in Salzb: erhalten werde, und das euch in Verwunderung setzen wird, unserm Sohne aber eine unsterbliche ehre macht.

Der nämliche Brief hat mir eine andre sehr angenehme zeitung mit gebrach.
[Briefe, i:246]

Yesterday I received a letter from Milan that notified me of a communication from Vienna that will be received in Salzburg, and will astonish you, but bring immortal honor to our son.

The same letter brought me another very pleasant piece of news.

The “sehr angenehme zeitung” (“very pleasant piece of news”) was Wolfgang’s commission to compose one of the carnival operas for Milan in the season 1772–73, the opera that became Lucio Silla. The “unsterbliche ehre” (“immortal honor”) was Salazar’s news that Wolfgang would be commissioned to compose the serenata for the imperial wedding in the autumn of 1771. The “Schreiben v Wienn” (“communication from Vienna”) has not been found.