The picture shows 3 pistons for the 2850 valve. The left side piston with the white plastic end cap is the standard hard water by-pass piston assembly #60105
which comes assembled with all piston parts.
This hard water by-pass piston will allow untreated water to pass downstream during the 80 minute regeneration cycle, if there is demand. Generally regeneration is set to occur during hours when there is little or no demand. This piston can also be used with a brine cam micro switch to turn off any motorized downstream equipment requiring water, thus eliminating flow.
The piston to the far right with the black plastic end cap is the 2850 No Hard Water By-Pass Piston (NHWBP) #60114-01 assembly. This piston will shut off all downstream water while the unit is in regeneration. This can be identified from the front of the valve by seeing the black plastic end cap. The square black plastic spacer fits between the brass casting and the back plate.
The center piston has a white end cap and triangular mounting frame with motor linkage attached. It is the standard 2850S piston assembly #61630-00.