Newer 2900s, Large Lower Piston Assembly, PN 61550, for the Fleck 2900s and NOT older 2900 valves.
This is the HWBP ( allows Hard Water By Pass during regeneration cycle ) version. Meaning it allows any water to flow past the valve while the system is in regeneration.
NOTE: Many "Commercial" applications require the NHWBP ( No Hard Water By Pass - PN 61555 ) version of this piston to prevent any water getting past the system while it is regeneration.
Lower Seals & Spacers sold separately ( 60128 ).
Top piston is the 61540 and Seals & Spacers kit 61530.