JC
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In the sources there is no 
before a1. It is related to different range of the octave sign at the beginning of the bar: the second semiquaver triplet is still not embraced by it, which makes that the on the level 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