M20 not wanting to go into reverse

[cwc]

I've noticed once or twice in the past year when I go to put my 67 ss427 in reverse it doesn't want to go,it's always when I first get in it. When I was leaving super car reunion it would not go into reverse at all,I figured I may have jerked it around too hard and knocked shifter out of alignment.i removed shift rod and would still not shift,I pulled trans,once on the bench it seemed to work fine,I went ahead and pulled tail shaft and found this tonight,this small ring was hanging out,I believe this was my problem,what purpose do these serve?