In the previous photo, the ring was complete, for one very important feature: We forgot to mill a clearance tunnel for the key to slide through, if need be. The wedge shaped, or ramped, key is supposed to become fast before it protrudes out the back side of the gear hub, but miscalculations do happen. much better to have the key stick through the entire gear, but still tighten up, than to hit the stop ring prior to wedging tight in the gear. In this view, it can be seen that the key has lots of room to freely pass through the stop ring.

Photo by Bill Liebman.