The report transcribed above comes from Staats- und gelehrte Zeitung des Hamburgischen unpartheyischen Correspondenten in Hamburg on 4 Apr 1783. It is equivalent (apart from a few minor differences) to a previously known report about Mozart’s academy published in Cramer’s Magazin der Musik under the date 9 May 1783 (Dokumente, 190–91). But the report in Staats- und gelehrte Zeitung precedes the version in Magazin der Musik by more than a month, and thus seems almost certainly to have been Cramer’s source.

The dateline of the report in Staats- und gelehrte Zeitung, “Schreiben aus Wien, vom 26 März” (“Letters from Vienna, on 26 March”), indicates that the story was submitted by a correspondent in Vienna, who might well have attended Mozart’s academy or at least had news about it from someone who did. The report states that the academy took place on “Saturday” (“Sonnabend”), namely 22 Mar 1783, also the date given in Cramer’s republication. This conflicts with the date that Mozart gives in a letter to his father on 12 Mar 1783, several days before the event, in which he writes that his academy will take place on Sun, 23 Mar:

Gestern hat meine schwägerin Lange ihre academie im theater gehalten, worinn ich auch ein Concert gespielt habe. — das theater war sehr voll; und ich wurde auf eine so schöne art von dem hiesigen Publicum wieder empfangen, daß ich ein wahres vergnügen darüber haben muß. — ich war schon weg. — man hörte aber nicht auf zu klatschen — und ich musste das Rondeau repetiren; — es war ein ordentlicher Plazregen. — das ist eine gute ankündigung für meine academie welche ich sonntags den 23:^t März geben werde. [Briefe, iii:259]

Yesterday my sister-in-law Lange gave her academy in the theater, at which I also performed a concerto. The theater was very full, and I was again received by the public here in such a lovely manner that it could only give me real pleasure. I had already exited, but the applause didn’t stop, and I had to repeat the rondo. It was a regular downpour. This is a good advertisement for my academy, which I’m giving on Sunday, 23 March.

In a letter to his father on 29 Mar 1783, Mozart describes his academy in detail, but does not mention its date or day (Briefe, iii:261–62). So we need to consider the possibility that Mozart’s planned date of 23 Mar was changed, and that the academy in fact took place on 22 Mar. The correspondent, writing after the event, may in this case be a more reliable witness than the composer, writing before it.

Mozart’s letter of 29 Mar includes a detailed program, and he mentions that the emperor sent him 25 ducats. (Assuming that the emperor sent imperial [kaiserliche] ducats, this would have amounted to 106 fl 40 kr at the time; on the conversion rate, see Edge 1991, 218.) The two concertos mentioned in Staats- und gelehrte Zeitung (and Magazin der Musik) were, according to Mozart’s own description, K. 415 (“das 3:^te von meinen Souscriptions=Concerten”; “the 3rd of my subscription concertos”) and K. 175, with K. 382 as the final movement (“das hier beliebte Concert ex D. wozu ich das variazion Rondeau geschickt habe”; “the Concerto in D to which I added the rondeau-variations, which is popular here”).