I treat myself to a pair of NAS lights for the LSE, I really fancied the extra running lights on the side and the original set were goosed.
I was dissapointed to find the yanks don't so fog lights, so the fog light is the stop light, without the ultra bright reflector and the side/stop lamp is just a side lamp. But wait, the bulb is a twin filament and if yo dismantle it then you can add/change the wiring to your hearts content, I even considerd having the fog lamp in the side lamp and the stop lamps seperate, but as the fog reflector (from the old lamp) was so much better I thought best not to.
So if the bulb is a twin filament then in some incarnation it was wired up.