At the end of November (probably the 22nd) 1968, both Paul McCartney and Donovan appeared at Trident Studio in London to work on Apple artist Mary Hopkin's LP eventually entitled "Postcard". The reason for the session seems to be that they were looking to cover an original song written by either of them in recent times.
Paul McCartney and Donovan both are playing together on acoustic guitars. Paul sings a song entitled "How Do You Do" which remains unreleased in commercial form. Paul also performs the current "Blackbird" for Donovan with Mary Hopkin joining in here and there. A third "Paul" song is introduced as "Heather" which is a musical nursery type song probably played to Linda's daughter for her entertainment.
Donovan plays "The Unicorn" which sounds like it's a very recent idea of a song. He also plays his current US single "Lalena" for Paul. Donovan completes the titles with three children's songs entitled "Mr. Wind", "The Walrus and The Carpenter" and finally "Land of Gisch".
As a side note, Mary Hopkin covered some tunes featuring both Paul and Donovan on acoustic guitars notably "Voyage of the Moon" and "Lord of The Reedy River" and the Donovan composition "Happiness Runs".
All of these acoustic tunes can be found on the vinyl bootleg "No.3 Abbey Road NW 8" although ironically none of it was recorded there, but at Trident.