The mono mixes of Good Morning Good Morning replaced those done on 6 April 1967. Fourteen mixes were made on this day, numbered 10-23, the last of which was used for the album.
The reason for redoing the mono mixes was because the song did not fit comfortably alongside Sgt Pepper's Lonely Hearts Club Band (Reprise). In the end the solution was to echo the final cluck of the chicken with the open guitar notes of Sgt Pepper (Reprise). The stereo mixes, which had been made on 6 April, merged the two together, diluting the effect.
The mono mix of Only A Northern Song was a demo done for reference purposes. Paul McCartney had recently returned from a trip to the US, and with recording on Sgt Pepper complete, was keen to press on with future projects.