The combination of these two adjustments provides almost a 2 stop alteration of the iris position in relation to the aperture ring.
Start by adjusting the sliding coupling on the left until the f/1.2 setting on the ring places the open aperture just past the field stop in the lens. The adjustment on the right is a max opening stop and can be moved if the ring doesn't center to the f/1.2 setting.
Fine adjustments can be made using sensitometry and a lens of known correct aperture setting on an evenly lit wall.