A lot of rosette-like diagrams from Opicinus' codex are decorated with the "calendrical letters" that repeat all around the rosette (sic: a b c d e f g a b c d e f g ...). One of the VMS pictures at Beinecke has a similar repeating pattern, but here the pattern is 17 characters long. Just a wild guess: 7 characters for the week days + 10 digits ?