The other light bulb is for the 'side marker' light used in Europe for on-street parking.
To operate them, you turn the light switch to the left.
1st click should operate the right hand side bulb and second click to the left operates the left hand side light.
This was a choice you could make depending upon where you parked the car.
These do not operate until the engine is switched off.
Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto