"The source for Othello
is Giraldi Cinthio's Hecatommithi. As the same collection contains a version of the story which provided the plot of Measure to Measure
, and as that play was written about the same time as Othello
, Shakespeare was presumably scanning Cinthio's book in his search for plots in the early years of the seventeenth century...in reading Cinthio's tale, he would have felt challenged by the magnitude of the difficulties involved in making a tragedy from it, but he would nevertheless have found several hints in the tale itself how the subject might be treated" (Kenneth Muir, The Sources of Shakespeare's Plays
With the binding being near contemporary, it would seem that the manuscript leaves bound in represent the remedy of an original fault, with it likely that these two gatherings were out of stock when this copy was issued.