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Source:
p. 6, b. 86-103
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In the sources there is no 
before a1 (in the case of JC and PE it concerns bar 66, repeated here as bar 102). It is related to a different range of the octave sign at the beginning of the bar: the second semiquaver triplet is still embraced by it, which makes that the at the pitch of a1 (in the notation) appears already before the 6th semiquaver of the bar.
Compare the passage in the sources»
category imprint: Source & stylistic information
issues: Accidentals in different octaves
notation: Pitch