Newer 2900s, Large Lower Piston Assembly, PN 61555, for the Fleck 2900s and NOT older 2900 valves.
This is the NHWBP ( Does NOT allow Hard Water to By Pass during regeneration cycle ) version.
Meaning it allows NO water to flow past the valve while the system is in regeneration.
Lower Seals & Spacers sold separately ( 60128 ).
Top piston is the 61540 and Seals & Spacers kit 61530.