In AI one can see that Chopin hesitated whether to write the B major section, which starts in b. 45, using a 4- or a 5-sharp key signature. It is evidenced by:
- a new key signature having been added in b. 45 (along with a double bar line at the end of b. 44) and in b. 57 (at the beginning of the page) and then the sharps of A
and a1 having been crossed out in both places; - the sharps before the a notes having been crossed out and then reintroduced (in various octaves) in b. 50, 52, 61 and 74;
- the naturals having been removed before a and a1 in b. 63.
The absence – with one exception – of such corrections of accidentals after b. 69 allows us to take a guess when Chopin decided to introduce a 5-sharp key signature. It may have happened at the time when he was writing b. 64-69, in which, despite the momentary return of the E major/C minor key, there are no a sounds that would require naturals. This could have tipped the balance, and this is the moment Chopin stopped writing sharps before the a notes (b. 74 with an introduced and crossed-out 
is most probably a moment of distraction) and added the change of the key signature and crossed out the already introduced sharps in b. 45-61. Eventually – in this case it is already difficult to determine the exact moment – the concept of change of key signature was abandoned, the sharps in the key signature – removed and reintroduced next to the notes (in b. 50-61) or added (from b. 70). The fact that sharps were being added in the latter is proven by the notation of b. 70 and 72 – wherever an a1 opens a bar, Chopin would not leave space for a sharp (cf. b. 92-94).
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category imprint: Corrections & alterations; Source & stylistic information
issues: Chopin's hesitations, Last key signature sign, Corrections of AI
notation: Pitch