23 in D major, Perger 43, Sherman 22, Sherman-adjusted 23, MH 287, is believed to have been written in Salzburg around 1779.
This symphony was incorrectly attributed to Wolfgang Amadeus Mozart by Ludwig von Köchel with the number K. 291, on the basis of a fragment of manuscript which Mozart copied, apparently to help him study the fugue form of the final movement.
Mozart did in fact copy out the first 45 measures of it (Simon Sechter completed the score of the finale[1]).
Some time afterwards, he wrote his String Quartet in G major, K. 387, with a finale that is also a fugato and also begins with a theme consisting of four whole notes first stated by the second violin.
Horns in D are used throughout, and in the fugal finale they even get to play the fugue subject a couple of times.