You`re missing the cupped/dished spring seat. A washer goes at each end of the spacer tube, then the dished seat goes over the top washer under the lock nut at the top of the damper rod.
This seat has a slot in it so you can slide it in place after the lock nut is tightened.
It`s a loose fit when the forks are disassembled, so it probably fell off at some point when you had them apart.
Look in the parts manual, part number 12 for the MC21 SP forks.