2-Pk. Sealand® Duck Bill Valves.
These babies prevent backflow! Allow pressurized water to pass through, but the "duck bill"-style end closes to prevent liquid from coming back through.
Sold in pairs. Fits model numbers: S12D, S12DAB, S24D, S32D, S115A, S12, 52A, S32, T12, T24 and T32.