Distributor o-ring ( 13/16" x 1-1/16" - 7170254 ) is # 7 in our diagram.
Fits on the top of the pipe in the resin tank, and the valve body slides over it.
North Star Commercial cross reference = PN 7133480
NOTE: Included in Oring Kit Valve to Tank - 7112963, shown as Related Product
Common o-ring to "fall off" and result in no soft water from a system that "appears" to be working normally.
Especially common on chlorinated "city" water, due to the effects of the chlorine on the oring, and the higher water pressures common on city water supplys.
Distributor O-ring Problem and Solution