Pentair Backwash Valve and Clean & Clear Cartridge Filters.
No need to change the whole valve, take the broken nut off and this will go right on. Fits the Newer Multi-Port Pentair valves also.
WARNING: The following product(s) can expose you to chemicals which are known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.p65warnings.ca.gov