Commentary
This play, appart from showing us an exchange of techniques, also provides a lot of information
Possible self-defence context - The whole play possibly starts in a self-defence situation, when the priest is just drawing his weapons. This is how I explain the phrase "custodia prima retinet contraria bina" (prima custodia has two opposistions). I believe it tells us, that a surprising attack will be performed by either a descending cut or a thrust.
The wrong reaction - defender should not react by performing in-tempo counterattack with an attempt to deliver a killing blow. The priest views this as either non-functional or too dangerous.
The correct reaction - The play tells us how to react to halpschilt siege (a downwards cut). The defender should fall under the sword and shield. If the attacker is a common fencer, he will go for direct thrust, that we will handle by stichslack (as shown in play 30 and play 31). The correct solution for the attacker would instead be a rebind.
How to handle the rebind - Against the rebind, the defender is instructed on the first page to perform a schiltslack. This is probably refering to the priest leaving the bind while checking the scholar's hands according to the play 38. On the second page, it is said that he can perform either mutatio gladii, durchtretten or back arms grab.
Prima custodia can also be besieged by another prima custodia.
Possible mistake
The scholar is said that after his rebind (he is then binding above and on the right), he may do either schiltslack or grab the priest's hands with his left hand. But the action of grabbing the priest's hands is never shown from this position and seems impossible.
The exchange shown
First two images of the play are showing us the situation and a reaction of falling under the sword and shield. In the next two images we follow correct solutions from the scholar (attacker) - he performs a rebind (that gets him into binding above and on the right) and then schiltslack.