Directional Gyro, Aircraft Instrument Manuf., 200DCFM Douglas uncages the compass card which

uncages the compass card which would then rotate right or left

Here's a helpful video on the Astrograph from a fellow enthusiast

The part number has been confirmed in the B-17F Erection and Maintenance Manual dated 1942 and the G model dated 1944

25 inches diameter and ~10 inches deep

Directional Gyro, Aircraft Instrument Manuf., 200DCFM Douglas uncages the compass card whichThis is a slaved Directional Gyro made by Aircraft Instrument Manufacturing for Aviation Safety, part number 200DCFM. Measures 3. 25 inches diameter and ~10 inches deep. The glass is clear and intact. The knob rotates the compass card to select the desired direction of flight.