A simple way to verify the magnesium hypothesis was to file away a small pile of dust from the mid-plane frame and...well...light it on fire.
Yup, looks like it's magnesium.
But why magnesium? If you take a look at your old pal, the Periodic Table, it is one of the lightest metals - more so than even aluminum. Additionally, its high strength makes it ideal for providing structural support where weight is a major concern.