It turns out there are different autographs (handwritten copies of “Bach’s original working manuscript of for the newly compiled collection the WTC II” that has been lost), that serve as source for those Urtext editions:
- the one of Johann Sebastian Bach and his wife Anna Magdalena (from 1739/40), today at the British Library Bach’s autograph in London
- Bärenreiter/Bach-Archiv Leipzig, for this specific aspect regarding this Fugue, refers to the autograph by Johann Christoph Altnickol (Bach’s student and later son in law, copied after Bach’s original in 1744), today at Berliner Staatsbibliothek Bach’s Autograph in Berlin
If you start digging into it a little the tradition of the WTC II in detail seems very fascinating…
If you follow the links above those autographs are beautiful to look at and read!
I am grateful to the Bach-Archiv Leipzig for replying to questions of this kind immediately.
Quote: Bärenreiter, The Well-Tempered Clavier II, Preface, Alfred Dürr/transl J. Bradford Robinson Further source, Bach-Archiv Leipzig