This O-ring is used on Clack BY PASS ( V3006 ), and all "Fittings" used with Clack valve or Bypass. Also Clack Valve Meter Plug ( V3003-01 ).Their part number for this o-ring is V3105 ( most commonly identified as the Distributor O-ring for Clack WS1 valves ).Also, common for many newer ( since 2003 ) MacClean / CUNO Valve body / adapters ( such as CEC 1000 )