In the sources the metronome marking reads 
=43. It must be a mistake, since the Maelzel metronome does not show this value. Assuming that only one digit is wrong, in the main text we suggest the closest number accessible to the users of traditional metronomes. We choose 42, and not 44, since confusing one of two identical digits is much less likely. Theoretically speaking, one could think of other numbers meeting the above criteria, e.g. 48 or 63, but it is already the first one that can hardly be considered a slow tempo.
Compare the passage in the sources»
category imprint: Interpretations within context; Editorial revisions
issues: Errors in FE, Errors repeated in GE, Errors repeated in EE
notation: Verbal indications