You can see the plate clearly in the earlier HQ parts catalogues, group 12.972. They call it "cover - floor pan gearshift lever (M) - floor shift" and say except for 37 bodystyles. The reason for the "except 37" is Monaro always had console shift with 4spd, all the other body styles had a floor shift version. I'm almost certain it is for installation clearance of the original HQ hockey stick shifter. It was probably deleted later when the shifter sticks became removeable.