Without ficta except LT at end
This version gives the passage without any ficta except for the cadence at the end, which seems certain to have had a raised leading note. This is the version endorsed by Ghiselin Danckerts, the theorist who related the story of the dispute concerning the ficta in this passage which occurred in the Papal choir ca.1540.
Phrygian mm. 6-7 + LT at end
This version shows how the bass singers took the Bb from their 3rd-last measure and applied it to the preceding cadence, making it a phrygian cadence. This created in incongruency between the superius-alto duo in mm. 1-4, and the tenor-bass duo in mm. 4-7, which have the same music an octave apart.
LT mm. 3-4, Phrygian 6-7 + LT at end
We know that the bass singers wanted to use flats and also wanted the alto and superius parts to follow their ficta, but that the singers of the upper parts refused. This caused the case to be sent to an arbitration panel in the Papal chapel (including the composer Cristóbal de Morales). This is one possibility for what the superius-alto singers sang if they refused to do a Phygian cadence, making a leading-note cadence in mm. 3-4 while the basses and tenors made Phrygian motion throughout.
LT mm. 3-4 & 6-7 + LT at end
This may have been what the superius and alto singers had in mind, making leading-note cadences in both mm. 3-4 and mm. 6-7 but, of course, we know the basses didn’t want to do this. The altos could argue, however, that the notated flat in the bass voice in m. 8 supports the non-use of a flat in the bass until that point.
Phrygian mm. 3-4 & 6-7 + LT at end
This version is presumably what the basses thought should happen, making Phrygian cadences in mm. 3-4 and mm. 6-7.
Oddly, we don’t know what the arbitration panel recommended. The most remarkable thing about the incident to me, however, is that the composer, Juan Escribano, was a member of the Papal choir at the time and no one asked his opinion. Ficta decisions were the right and responsibility of the performers, not the composers.
