There is a very unusual feature to this concerto.
In addition to omitting the tempi for two of the movements, Mozart also, in Tyson's words, "did not write any notes for the piano's left hand in a great many measures throughout the work."
As can be seen in the Dover Publications facsimile, large stretches of the solo part simply have nothing at all for the left hand, including the opening solo (mvmt.
I, mm.
81–99) and the whole of the second movement.
There is in fact no other Mozart piano concerto of which so much of the solo part was left unfinished by the composer.

The 1794 first edition had these gaps filled in, and most Mozart scholars such as Alfred Einstein and Alan Tyson have assumed that the additions were made by the publisher Johann André.
Einstein is on record as finding André's completion somewhat wanting: "For the most part, this version is extremely simple and not too offensive, but at times—for example, in the accompaniment of the Larghetto theme—it is very clumsy, and the whole solo part would gain infinitely by revision and refinement in Mozart's own style."

Nearly all of the passages that necessitated filling in for the first edition lack only simple accompanimental patterns such as Alberti bass figures and chords.
For example, measures 145–151 of the first movement, which involve more complicated virtuoso passagework, are fully written out in the autograph.
For the less complex portions of the solo, it is clear that Mozart "knew perfectly well what he had to play" and so left them incomplete.

The old Breitkopf & Härtel Mozart Complete Works score of this concerto does not make any distinction between what Mozart himself wrote and what André (or someone commissioned by him) supplied.
However, the Neue Mozart-Ausgabe volume referenced above prints André's supplements in smaller type, to clearly distinguish them from Mozart's own notes.
There is a very unusual feature to this concerto. In addition to omitting the tempi for two of the movements, Mozart also, in Tyson's words, "did not write any notes for the piano's left hand in a great many measures throughout the work." As can be seen in the Dover Publications facsimile, large stretches of the solo part simply have nothing at all for the left hand, including the opening solo (mvmt. I, mm. 81–99) and the whole of the second movement. There is in fact no other Mozart piano concerto of which so much of the solo part was left unfinished by the composer. 
The 1794 first edition had these gaps filled in, and most Mozart scholars such as Alfred Einstein and Alan Tyson have assumed that the additions were made by the publisher Johann André. Einstein is on record as finding André's completion somewhat wanting: "For the most part, this version is extremely simple and not too offensive, but at times—for example, in the accompaniment of the Larghetto theme—it is very clumsy, and the whole solo part would gain infinitely by revision and refinement in Mozart's own style."
Nearly all of the passages that necessitated filling in for the first edition lack only simple accompanimental patterns such as Alberti bass figures and chords. For example, measures 145–151 of the first movement, which involve more complicated virtuoso passagework, are fully written out in the autograph. For the less complex portions of the solo, it is clear that Mozart "knew perfectly well what he had to play" and so left them incomplete. 
The old Breitkopf & Härtel Mozart Complete Works score of this concerto does not make any distinction between what Mozart himself wrote and what André (or someone commissioned by him) supplied. However, the Neue Mozart-Ausgabe volume referenced above prints André's supplements in smaller type, to clearly distinguish them from Mozart's own notes.
Piano Concerto No. 26 (Mozart)