The part w/ the model name and Japan on it is plastic. That is the backside of the housing and all the Sonys have plastic on the backside which you should be able to see from my pics. The 600 is the only one w/ plastic on both sides. The 600 is aluminum, 1000 magnesium, 7550 somewhere in between? 7550 has a multi layer polymer diaphragm. Sony's way of not saying we use the same diaphragm as the EX1000 (liquid polymer).