This is an Airplane Headlight bulb. Indeed, airplanes do have headlights, they use them every time they land at night. They are usually mounted in the wings nearest to the fuselage, or on the nose landing gear. This particular bulb could very well be a WWII bulb, for use in a Douglas DC-3 or "Skytrain". Out of all the airplane lightbulbs I've seen, none was ever stamped "airplane headlight". You know, those two words stamped on top make the bulb a whole lot more interesting!