dual repalcement cranks

The opposite-handed counterpart of the 39-156 is the 39-155. Traditionally this set hasn't been specifically designated for left and right-handedness, since this designation actually differs across windows. But in the case of this set specifically, the side of the spline that the gears are concentrated on and from which the dual-arms emanate is what determines the 'handedness' of your old operator--the 39-155 has these gears on the 'left' side of the spline, while the 39-156 has these gears on the 'right' side. Let me know if this makes sense or if you have further questions.