In the sources one or both naturals before the a1-c2 third were omitted. Lack of the 
before a1 (in all sources) is a patent error. What is interesting is the fact that the defect was not corrected in GE and EE4 despite adding a cautionary before a2. In turn, lack of the returning c2 in Ap, although most probably incidental, leads to a version with c
2, which, from the harmonic point of view, is not impossible.
Compare the passage in the sources »
category imprint: Interpretations within context; Differences between sources
issues: Omissions to cancel alteration
notation: Pitch
