I am rebuilding a pair of pumps from parts I have acquired over the years. One of the SU marked rubber diaphragm has a sheet of thin clear plastic covering the the rubber on the fuel side of the diaphragm. It appears the plastic & rubber have been punched out at the same time including the holes & then riveted to the spindle. Does one keep the plastic or remove it prior to assembling the pump? It just seems strange that you would remove it with the possibility of damaging the rubber diaphragm.