Identifing a wire

Thanks for your help! It seems it is meant to be there then if you’ve also used a jumper wire; Could you possibly enlighten me on what the reserve lighting device is for?
Switches hi beam on at a reduced level if the dipped beam bulb blows. Or summat like that. I deleted it just because it's 47 years old and I don't trust it. Besides the bike will not be used when it's dark anyway.