Jim, the 1940 operating manual lists bulb 616-13 for 001, 002, 003, 004, which is a small, screw-base 12 v clear bulb. Your 18 v bulb must have been defective, as it has a higher rating than the recommended 12v bulb. Any 12 to 18 v small screw base bulb should work, but an 18v bulb will burn dimmer than the 12v. Hope this helps.
(also) Jim