The Final Rinse Piston is only compatible with the following units:

• CSI Nitro, Nitro Pro, and WaterSoft Centuri filters manufactured after 6/1/2015 (or ones that have previously been upgraded to the final rinse piston).  
• Any CSI Sidekick
• Any Water Soft Sidewindr

If your unit uses the Final Rinse Piston, it will have a red mark on the top, as shown in the photo.  If your unit uses the standard piston (no red mark), its circuit board is not compatible with the Final Rinse Piston, so you will either need to use the 20001X231 (standard piston assy) or upgrade the electronics to be Final Rinse Piston compatible by changing the powerhead, or installing a new board & motor kit.

Anytime you're replacing a piston, we recommend installing a new seal & spacer kit.