The manuscript score is lost, but from the latter part of the 20th century onwards many performances of the work have been given on basset clarinets in conjectural reconstructions of Mozart's original.

Anton Stadler, a close friend of Mozart, was a virtuoso clarinettist and co-inventor of the basset clarinet, an instrument with an extended range of lower notes.

[5] In early October 1791 Mozart wrote to his wife from Prague that he had completed "Stadler's rondo" – the third movement of the Clarinet Concerto.

[8] This excerpt, dating from late 1789, is nearly identical to the corresponding section in the published version for A clarinet, although only the melody lines are completely filled out.

Objections were raised to this: a reviewer in the Allgemeine musikalische Zeitung commented that although the transpositions made the work playable on standard clarinets, it would have been better to publish Mozart's original version, with the alterations printed in smaller notes as optional alternatives.

[20][21][22][23] The most frequently performed are the two versions published by Bärenreiter (Kratochvil/Hess) and Schott (Wehle/Meyer)[24] In 2023, the clarinettist Richard Haynes undertook to rewrite the score in G major using the fragment K. 621b.

The form of the movement is as follows: The first theme begins an orchestral ritornello that is joyful and light: It soon transforms into a flurry of sixteenth notes in descending sequence, played by the violins and flutes while the lower instruments drive the piece forward.

[28] The canonic material of the opening ritornello returns, this time involving the clarinet and leads to the novel feature of the soloist accompanying the orchestra with an Alberti bass over the first closing theme.

Before the formal orchestral ritornello leading into the recapitulation, Mozart writes a series of descending sequences with the cellos and bassoons holding suspensions over staccato strings.

As is conventional in Classical concerto form, in the recapitulation the soloist and orchestra are united, and the secondary theme is altered to stay in the tonic.

The Alberti bass and arpeggios over diminished chords for the soloist recur before the movement ends in a cheerful final orchestral ritornello.

This section is followed in bar 59 by the aforementioned cadenza, which in turn is followed by the first theme, played piano (sometimes pianissimo) by the clarinet, which concludes the movement with a coda.

[citation needed] The first recordings of the reconstructed counterpart were made in September 1968 with the Swiss clarinettist Hans-Rudolph Stalder [fr] and the Cologne Chamber Orchestra, conducted by Helmut Müller-Brühl.

For this and for concert performances, Stalder had a modern Boehm basset clarinet built at the F. Arthur Uebel factory in Markneukirchen, Saxony.