From the auction house's press release:
In 1977, Gandy deposited the papers at the Archive Center at King's College, Cambridge, Turing's old college – where they have been available to scholars for research ever since. He did, however, retain one item – this manuscript. In the blank center pages of the notebook between Alan's writing, Gandy wrote his dream journal. The contents of the journal are intensely personal, so it is not a surprise that he would want to keep the journal private, and in fact, it remained hidden amongst his personal effects until his death. As he wrote at the beginning of this journal, "It seems a suitable disguise to write in between these notes of Alan's on notation, but possibly a little sinister; a dead father figure, some of whose thoughts I most completely inherited."