This is an Airplane Headlight or Landing light bulb. They may be marked Airplane Headlight, Airplane Landing, even simply "AVIATION". Indeed, airplanes do have headlights, they use them every time they land at night. They are usually mounted in the wings, often nearest to the fuselage, or on the nose landing gear. This particular bulb is more than likely a WWII bulb, for use in a Douglas DC-3 or C-47 "Skytrain". Out of all the airplane light bulbs I've seen, none was ever stamped "airplane headlight", just this one. You know, those two words stamped on top make the bulb a whole lot more interesting!