The natural raising a
to a is written in A with pencil. The sign clearly differs from both typical Chopinesque naturals and the signs added in bar 314 (as well as from the 
written with pencil in the part of the clarinet I in Morch in this place). Moreover, a denser text in GE1 suggests that in this edition a corresponding sign was added only in the proofreading. Therefore, it may be one of the later, foreign additions – cf. bar 256.
Compare the passage in the sources»
category imprint: Interpretations within context; Differences between sources
issues: Foreign hand additions in manuscripts, Omission of current key accidentals, Errors of A, Authentic corrections of GE
notation: Pitch