The date of completion of this symphony is known exactly, since Mozart in his mature years kept a full catalog of his completed works; he entered the 40th Symphony into it on 25 July 1788. Work on the symphony occupied an exceptionally productive period of just a few weeks during which time he also completed the 39th and 41st symphonies (26 June and 10 August, respectively). Nikolaus Harnoncourt conjectured that Mozart composed the three symphonies as a unified work, pointing, among other things, to the fact that the Symphony No. 40, as the middle work, has no introduction (unlike No. 39) and does not have a finale of the scale of No. 41's. The 40th symphony exists in two versions, differing primarily in that one includes parts for a pair of clarinets (with suitable adjustments made in the other wind parts). Most likely, the clarinet parts were added in a revised version. The autograph scores of both versions were acquired in the 1860s by Johannes Brahms, who later donated the manuscripts to the Gesellschaft der Musikfreunde in Vienna, where they reside today.