The third variant for the parry of Tertia is a bit unclear. As best as I can tell, you parry just like we did in part 1. But instead of riposting over the arm in Tertia, you riposte under the arm in Quarta.
If I’m correct, then he is referring to illustration 53, which was described in chapter 7.
While this illustration isn’t explicitly referenced in this chapter, he again warns that the off-hand is needed to prevent a counter-attack.