A similar quadrant appears on the shelf at the top left of Holbein's 1528 portrait of Nicholas Kratzer:
A second paper quadrant rests behind the wooden one.
Foister includes the following useful discussion of these quadrants:
Such quadrants were used for determining the angle and the altitude of the sun from the horizon, from which it was possible to calculate the time. To do so they would use a plumbline, a small lead or brass weight on a string which would hand straight against the marked segment when the instrument was raised towards the sun. This is clearly visible to the left of the nearer instrument. However, the further quadrant is either incorrectly assembled or incorrectly depicted, and would have been impossible to use --for the sights, the small tab-like objects on the right-hand side-- are placed on the wrong side of the quadrant, and the whole object appears as if upside down*