That is the first thing to go onto the drive and jackshafts. The case seals ride on the metal ring, and the o-ring seals the shaft. Put it on with the o-ring facing you. Just make sure the metal ring is pushed all the way on and the o-ring is all the way inside that recess area. http://parts.polarisind.com/Browse/Browse.asp
follow this link and look your sled up. go to drivetrain section and you'll see it.