O-ring ( 15/16" x 1- 3/16" ) used on installation adapters and standard size By Pass Valves on most Sears, North Star, GE, and Ecowater systems.
part numbers 7170288, 900535, STD302213, WS03X10025
You should order in pairs ( 2 required ).
Also, one used on larger Drain Adapter ( 7172793 )
Also, one used on the Guide Cap of Brine Float Assembly ( 7310197, label # 36 in our diagrams ).