From the thumbnail it looked like a Form109...........and don't be surprised if it never falls off, I have seen some that never fell off! I know a Ballast in Arm in Rockville that hasn't lost the housing but is in the same situation as above! I have a pic of it!
Yeah...also I've kinda seen when one hangs like this, sometimes it stays becauase the lamp is "in the way" from letting the reflector move more which is why sometimes I see them still staying longer......