The 446 incandescent bulb was original equipment on various BSA, Norton and Triumph models and is used in glass reflector units with the British Pre-Focus receptacle.
The genuine Lucas 50/40 dual filament bulb features a dual-point base that is secured in the receptacle with a BPF bulb holder.
The 50 watt high beam and 40 watt dip beam is normally switched through the high/low/horn handlebar switch.
The bulb is dropped into the receptacle on the back of the head lamp glass light unit and secured by the BPF bulb holder, which has tabs that are located into the notches in the receptacle. The bulb holder is turned 1/4 turn to lock in the bulb.