For Sale is (1) New Old Stock (NOS) SRAM X-0 Rear Derailleur.  It's designed for 3x9 Mountain Bike drivetrains, has a 1-1 actuation ratio, and has a Carbon Fiber Long Cage.  It is in MINT Condition.  In my opinion, it's the finest 3x9 speed Mountain Bike Rear Derailleurs.  Reliable, great looking, and precise shifting.