PISTON ROD, only, Fleck 5600, PN 13001.
It is a special order item ( 2 week lead time ).
And problems with Piston leaking from the rod are best solved by replacing the entire piston.
The "leak" is a combination of wear on the Teflon coating, and the internal Quad O-ring in the piston CAP.
And if you have issues with either, that usually means it's time to replace the entire piston assembly.
To learn more about the 3 common types of Pistons, see:5600 Piston Assembly Types - Softener - Filter - Low Water, Piston Assembly 60102-00, 60102-10, 60102-20,
What is the Difference?