This is a General Electric NE-40 neon bulb, similar to the NE-34 but with two complete metal disk electrodes rather than a single disk split in two. These bulbs were sold to be used as night lights and safety lights for exits and alarms, etc.; The NE-40 and NE-34 bulbs are much harder to find than the similar AR-1 Argon glow bulb, which is an easy find.
This glow bulb does not have an actual "filament", but rather a pair of disk-shaped electrodes, so it cannot "pop" when power is applied to it. Glow bulbs like this do not fail all of a sudden, but rather gradually lose their ability to glow, often with continuous flickering at the end of their useful life.