These are replacement lids for Racor's series of Turbine Filters.
There are smaller lids for the 500 series, and larger lids for the 900 or 1000 series (which are the same size).
Some specialized lids are equipped with vent ports.
Note: Confused by the FGs and FHs?
Over the years, Racor has gradually made modifications and improvements to the F Series of Turbine Filters. Each new version has been designated by a letter suffix at the end of the model number. Recent versions include FE, FF, FG, and FH filters.