This is a pre-WWII-era Gyro Horizon (Flight) Indicator, Army Type C-5, made by Sperry, part number 643770, marked for the Air Corps US Army. This indicator has a serial number prefix "AC-40", evidence of a manufacturing year 1940, prior to the US entering WWII. The C-5 Gyro Horizon was used across all aircraft of the US Army Air Corps, including bombers B-17, B-24, B-25, fighters P-38, P-40, P-47, P-51, and transport C-47.
The Gyro Horizon provided visual cues to the pilot regarding the attitude of the aircraft, i.e., banked port or starboard (and by how many degrees) and diving or climbing. The caging knob at the lower right corner would either capture the gyro in a fixed position i.e., caged, when encountering turbulent motion which could tumble the gyro, or release it for use during flight. The gyro is vacuum-powered, receiving its suction from either a engine-powered vacuum pump or externally-mounted power venturi tube.
The center knob adjusts the center 'airplane' silhouette up/down. This corrects for the height of the seated pilot by aligning his line of sight with the silhouette and the tilting horizon marking behind it while in level flight. Otherwise, a tall pilot might want to climb his aircraft that is actually in level flight, since he would view the silhouette as being positioned below the horizon and think he is in a dive. Vice versa for a short pilot.
This indicator is experienced but in good condition for a 70yr+ old artifact. The knobs are stiff but rotate. Measures approx 5 inches diameter and 8 inches deep.