Today IMSLP posted Mozart's Fantasy in C minor, KV 396. Most of KV 396 was not composed by Mozart, but there is no mention of that on IMSLP.
KV 396 is a completion, made after Mozart's death, of an unfinished sketch, KV 385f, by Maximilian Stadler. Ostensibly, Mozart intended KV 385f for violin and piano. He composed only 27 bars of music, up until the repeat barline. Stadler turned Mozart's unfinished sketch into a piano piece, which is what IMSLP posted. The entire second half of the Fantasy, which is much longer than the first half, was composed by Stadler. It is not only mediocre music, it has very little musical connection with the 27 bars that Mozart composed (which, though incomplete, constitute one of his great adagios.)
The extent of Stadler's completion is thoroughly documented in the Neue Mozart Ausgabe for KV 396. I imagine it is also documented in the old Peters edition, which you posted -- but if it is, identification of Stadler's work does not appear in the posted portion of the published score. Nor does it appear on IMSLP's information page for this piece. It would be a shame if any unsuspecting viewers thought that Mozart composed Stadler's artless second half of this movement.
MS
Most of KV 396 not composed by Mozart
Moderator: kcleung