Fleck 60090 water softener piston assembly. Fits the Fleck 1500, 2500, 2510 and the 2700 control valves. This is the complete piston assembly, some manuals break it down into separate parts.
The Fleck piston assembly is easily changed. Any time you change a piston, it is always recommended to replace the seals and spacers (60121 https://watersoftenerpart.com/product/60121/) as well.