The seal drives into the housing. I tapped mine in with a deep 1/4" drive socket. Let me look in the manual to see if it tells. I remember doing a gear and seal change but I don't remember how the ring worked. It does look like it snaps in the housing.
Nothing in either of my manuals about the snap ring or seal other than replace the seal if there's fluid in the speedo cable when you unscrew it from the adapter.