In (6a), the assumption is that there is no lengthening, but we may argue that the final vowel is lengthened on a high tone, following Bamgbose's (1980) suggestion that the high tone syllable is assimilated to the final vowel of the subject.
However, while Bamgbose sees the lengthened vowel (or the High tone in cases where assimilation has taken place) as a concord marker, Awobuluyi sees the lengthened vowel as a High Tone Syllable (HTS), which may be assimilated to the final vowel.
(The traces of the original splits are thus seen in separate ru tone mergers and the shang tone syllables
that shifted into qu tone.) We are left with the uneasy feeling that the reason the book does not clearly explain all this is because the author himself does not understand it.