Toro 20622 (809.. SN) Mower Drive Wheel Issue

This old mower is has a left rear wheel that loses it self-propel drive about once/season.
 
The locking ring pops off somehow and the rocking key, spring, washer and sometimes gear vanishes into the lawn.
 
Before I order all those parts for another repair, I want to ask if there is something else that is causing the snap/locking ring to fall off or a way to secure it better.