@aaronm@medievodons
That cross figure is a variant (with extra letters) of a figure poem by Ps.-Venantius Fortunatus, which in turn is based on a couplet from the middle of a North African church inscription composed by Calbulus and found in the Codex Salmasianus (Anthologia Latina). There are several witnesses.
I would love to correspond with anyone researching this, as I am. It's an example of a "labyrinth," best known from the Tabulae Iliacae (1st c. BCE).