Certikin suction cover
This grille, which was marketed under the Certikin brand, has not been manufactured since around 2008, as at it did not comply with the American "VGB" pool and spa safety law, passed that year.
You can replace it with the Hydroair suction cover, which uses the same thread.
Both the old and new grilles screw into a HydroAir wall fitting.
Dimensions
Face diameter: 86 mm
Height: 28 mm
Thread diameter: 45 mm
Part numbers
Certikin: HD54, HD54/N4