I don't know exactly what's in there, but it's essentially something to keep the light from firing with very low amounts of current. In pinball machines there's often low amounts of current running through the game, even when the light isn't on. Incandescent bulbs don't light up because it's not quite enough current to do so, but LEDs will flicker on a bit with this small amount of power. The non-ghosting LEDs have something in the base to raise the amount of current needed to make it fire above the threshold of that small amount of current that is often found running through the game. It's most likely a resistor.