The Honeywell 50000293-004 is the correct replacement post filter for the following air cleaners:
The Honeywell 50000293-004 Post Filter 12.5x20 2-Pack is a replacement post filter for the 20x25 and 20x12.5 F300E and F50F air cleaners. The Honeywell post filter increases the efficiency of your Honeywell electronic air purifier model and has only a minor effect on its pressure drop. This Honeywell media air filter is charged and boasts high efficiency.
Honeywell 50000293-004 Post Filter 12.5x20 2-Pack specifications: