The purpose of the black roller with offset screw is to adjust the pick up point for the carbs' throttle opening in relation with the cam. Which in turn is tied to the timing tower that is controlled by the control cable. The timing tower controls both the advancing of the timing (through the trigger plate) and also the opening of the carbs throttle (through the cam and roller and tie bar for multiple carbs). And both movement should be in synch.
At mechanical zero, i.e. carb throttles fully closed, idle screw backed off the engine block, and timing at zero, the black roller should be in contact with the cam at its hash mark. If not, then by loosening the locknut on the roller, you can adjust the eccentric screw to make it so. Usually, the slot of the eccentric screw will also be in line with the cam's marking.
Of course, as soon as you set your idle speed, the cam's marking will be a little bit lower than the roller, indicating the carbs' throttles are slightly open hence also the timing.