A peculiarity of the book is that the bass lines were printed beneath the treble, upside-down! The book was to be placed on a table between the musicians and read from their perspective. A later edition reversed this, and printed the all the parts so they can be read from a shared music stand. The 1988 edition includes an image of the original Page 12, which illustrates this, and also some of the problems posed for modern scholars and musicians.
The 1988 edition contains third (alto) parts for all tunes, but there seems to be no clue about their origin. They may have been written by the editors (Christine Helwig and Marshall Barron).