If I remember correctly the cover was fitted to stop any large/thick walled fuel pipe blocking the vent hole. The cover had a raised ridge running down its centre to provide the ventilation.
Attached is a schematic of the vent cover as I have been unable to find a satisfactory photo/illustration of the cover.
The fuel inlet pipe fitted through the hole in the vent cover and was stopped from rotating by the side walls. It was kept in place by the fuel pipe.[frame]

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