Nut used on 1650 Brine Valve Assembly GFN 19625 ( Includes parts labeled A, B, and C in the diagram ).
The BLFC fitting and Brine Valve ( 1650 ) are normally re-used.
Or they can be purchased separately.
Also available Separately - if needed is Insert, made of brass ( 10332 ) used on 3/8" O.D. Brine Line and Brine Tubes

This plastic Nut is used with Brine Valve Assembly ( 3/8" tubing size not 1/2" size ) is used on the newer Fleck 2510, 2750, and 2850 Valves.

Also used with 3/8" Brine Shut off 2310 ( 60014 ).