The flats added in FESf could not have been written by Chopin, since the copy of FESf comes from an impression released sometime after his death. According to us, the variant entered into FESf may be, however, authentic: the pupil could have written down a change indicated by Chopin in a copy he purchased later.
A stylistic analysis leads to the conclusion that Chopin could definitely have suggested such a variant, since he would willingly use a Neapolitan chord; moreover, in this specific place, the C major chord is an exact equivalent of the G major chord from b. 546, so the version of the progression in FESf sounds as naturally as the version of the remaining sources.
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category imprint: Differences between sources
issues: Annotations in teaching copies, Annotations in FESf
notation: Pitch