The harpsichord part in the autograph manuscript is not a "fair copy" but a composing score with numerous corrections. The orchestral parts on the other hand were executed as a fair copy. Various possible explanations have been proposed as to why Bach assembled the collection of harpsichord concertos at this particular time. One centres on his role as director of the "Collegium Musicum" in Leipzig, a municipal musical society, which gave weekly concerts at the Café Zimmermann, drawing many performers from students at the university. Bach served as director from spring 1729 to summer 1737; and again from October 1739 until 1740 or 1741. John Butt suggests that the manuscript was prepared for performances on Bach's resumption as director in 1739, additional evidence coming from the fact that the manuscript subsequently remained in Leipzig. Peter Wollny, however, considers it unlikely that The "Collegium Musicum" would have required orchestral parts that were so neatly draughted; he considers it more plausible that the manuscript was prepared for a visit that Bach made to Dresden in 1738 when he would almost certainly have given private concerts in court (or to the nobility) and the ripieno parts would have been played by the resident orchestra. Peter Williams has also suggested that the collection would have been a useful addition to the repertoire of his two elder sons, Wilhelm Friedemann and Carl Philipp Emanuel, both employed as professional keyboard-players at the time of writing. Williams has also speculated that it might not be mere coincidence that the timing matched the publication of the first ever collection of keyboard concertos, the widely acclaimed and well-selling Organ concertos, Op.4 of George Frederic Handel published in London and Paris in 1738.