Flipping the lens again, make a solid mark on the front aperture retaining plate and the barrel of the lens. This mark is imperative to get the aperture lined up correctly in the future.

If this mark is not made, you may end up with inaccurate f-stops.

Remove the screw marked in orange. Not pictured: two other screws offset 120 degrees that should be removed as well. Then remove the black retaining plate.