That doesn`t look like a "bulb" to me. Reckon it`s actually a heater - or at least - was. There look to be marks on the upper greeen ceramic portion where an insulated heating element would have been coiled around, and connected to the screws at the bottom. Such things would have normally been used screwed into a fixture with a parabolic metal reflector to direct the heat where needed. An interesting curio but probably no longer functional.