Not a double ring. An extra ring added to the tube I guess. If you lok at the JB on their site and the las one has extra ring in the middle
That may be for PA anchoring or extra security, or for no other reason than it's a cool thing. It may be because that cage is longer than most. Why not message Mistress Mature Metal (site member) and she can answer all your questions about MM devices
My Goddess preferred how the extra cage ring looked for extra security. MM's products are incredibly rigid and doubt the extra ring is needed for strength but sure does not hurt - I also like the look of the extra ring and it adds some tightness in the middle if the cage. FWIW I also added the double base ring option - I've been locked in my JB for 14 months now, released on weekends for play. Asides from a brief problem with an abrasion that opened up that was solved by a small 1/16" gap adjustment it has been an awesome experience being locked away.