Ding Ding Ding we have a winner!
You are correct Steve Tow!
1. Its called Coil Spring Stop
2. Ìt stops the sliding head from falling off the pipe.
3. It goes on the threads on the tail stop / sliding head end of the pipe.
Steve Tow you can collect your prize from munchy or Brian.
Jeff Vandenberg aka "Woodsconsin"