Brass Nut ( 10329 ) used on 3/8" O.D. Brine Line and Brine Tubes.
Many Fleck valves use this brass nut on the 3/8" connection common with residential water softener valves.
** NOTE: You will usually NEED a new FERRULE ( 10330 ) when replacing this Nut. Ferrule plastic Delrin Sleeve (3/8 Tube) PN 10330