That "key" is not missing- - - -it's broken, which probably is also the reason for the damaged corners of the slots in the brass ring. The annular rings that actually grip the taper on the gear and match the speed during a shift are about halfway worn out- - - -they should come to a sharper point. That ring you found in the case is actually a spring that fits into a groove in those keys and pushes them out into the sliding shift collar. Bottom line- - - -you need a complete synchronizer assembly, or at least new keys, a spring, and a new blocker ring. Those parts might be a bit hard to find unless you can scavenge up a good used synchronizer.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!