I have added the following entry to the Alto Trombone History Timeline and the Trombone History Timeline (18th century). It involves specific mention of alto trombone in a letter from Leopold Mozart to his son about a work by Michael Haydn.
1777—Salzburg, Austria: In a letter to his son, Wolfgang, Leopold Moazrt writes the following about a performance of Michael Haydn’s Missa Sancti Hieronymi: “I liked the whole mass very much, as there were six oboists, three double basses, two bassoons and the castrato who has been taken on for six months at one hundred gulden a month. Ferlendis and Sandmayr played the oboe solos. The oboist at Lodron’s, a certain student, the chief watchman and Oberkirchner were the oboists in the orchestra. Cassel and the choirmaster Knozenbry were the double basses, seated near the organ and beside the trombones; Estlinger was there with his bassoon; Hofner and Perwein were seated beside the oboists on the violinists’ platform. What I particularly liked was that, since oboes and bassoons resemble very much the human voice, the tutti seemed to be a very strongly supported chorus of voices, as the sopranos and altos, strengthened by the six oboes and the alto trombones, admirably balanced the number of tenor and bass voices; and the pieno was so majestic that I could have easily done without the oboe solos. The whole affair lasted an hour and a quarter and I found it far too short, for it is really an excellent composition” (Anderson, The Letters of Mozart and His Family, 352). For a slightly different translation of the letter that also uses the term “alto trombones,” see Karl August Rosenthal, “The Salzburg Church Music of Mozart and His Predecessors.” The Musical Quarterly, Vol. 18, No. 4 (Oct., 1932), 559-577. According to Trevor Herbert, “In fact, the cover of the autograph score lists ‘3 Tromboni tutti obblig.’” (Herbert Trombone, 352).