the four corners of a circular volvelle

Hmm . . .

(Guild Book of the Barber Surgeons of York, late 16th-century; British Library, Egerton 2572 f.51; a zodiac volvelle surrounded by John the Baptist with Agnus Dei, John the Evangelist with a cup and dragon, Cosmas with a urine file, and Damian with a medicine box. Image from the British Library.)

. . . but then . . .

(Our lovely friend the Denckring, from Georg Philipp Harsdörffer, 1651.)

. . . ?

There's something about those four corners around a volvelle on a rectangular page . . . and what fills them . . .