Assembled iris is carefully guided into lens vertically with the lens upside down.
The iris pin must engage the fork in the aperture linkage.
Insert the pin into the curved slot, the fork is on one side.
Turn the lens over and wiggle the iris a little to seat.
While holding the iris in place, move the aperture lever on the other side
and check for smooth operation.
The iris should snap back closed smoothly when the actuator is released.