to mention only two aspects of the development - speak not only to Berger's thesis concerning the emergence in Italy of the concept of equal breve across all mensurations but also to the complex history of quaternary subdivisions and their role in spawning proportional notations, both of them topics of central concern in this book.
I would conclude that the mensuration signs in this diagram are simply decorative, not meaningful.
The discussion of the rhythmic structure of three of these mensurations, and the consequent tie between rhythmic structure and the predominant value that carries the text, are unproblematic.
For each phrase category, besides a discussion of its characteristics and the statistics of the text underlay, Boone provides carefully commented examples in the different mensurations, putting particular emphasis upon those cases where the scribal placement of the syllables is relatively ambiguous.
All settings except the final Magnificat are notated in the conservative [cents] mensuration
emphasizing semibreves, minims, and semiminims, with triple-meter passages of semibreves in 3/2.
The question of the relation between the two mensurations
is not directly relevant (though it surely has implications for the speed of O everywhere else in the Mass).
All note values are halved, Mensurstrich is used throughout, and changes of mensuration
sign during the course of a piece are indicated above the staff.
This change dropped the tempos in all mensurations
to 3/4 of what they were before.