...for the purpose of mounting a front lamp when there is no more room for it on the handlebar itself.
The mounting is done on the vertical tube which holds the handlebar. Two screws M4x16 and two self-locking nuts (M4) are required.
I apologize for not translating the variable identifiers in the Customizer. D1 means the diameter of the vertical tube, D2 is the diameter of the additional handlebar rod.