Leopold Mozart Alto Trombone Concerto Second Movement - Adagio.
Extracted from Leopold Mozart's nine movement "Serenade" of 1755, three movements of which are original for trombone. The solo trombone part was certainly written for Thomas Gschlatt, who was a member of the Salzburg court chapel from 1756 to 1779. Leopold Mozart may have added the annotation after Gschlatt left Salzburg, because he knew that the chances of coming across another trombonist of Gschalatt's calibre were slim indeed. Watch the trombonist Ricardo Mollá performing this piece through this Youtube link.
Movements are entitled Allegro (2/4), Adagio (2/4) and Menuetto (3/4).