If FCI accurately represents the notation of [AI], the stimulus to begin the crescendo was initially the five-quaver ascending L.H. sequence in b. 20. According to us, this version may be considered an acceptable variant; on the basis thereof, one can also shape accordingly the course of the crescendo starting – in accordance with the main text – in b. 19.
We believe that the placement of those indications – over the top stave (in A) or under the bottom one (in FCI) – results from lack of space between the staves; in the case of A the reason were also the crossed-out 
and 
hairpins, which were initially written there (see the next note).
Compare the passage in the sources»
category imprint: Differences between sources; Corrections & alterations; Source & stylistic information
issues:
notation: Verbal indications