I previously spent some time pondering this question since metallurgy is a small portion of my real world employment. It would seem a really bad idea NOT to make it of stainless steel since undoubtedly it would rust around the edges or if scratched through the plastic coating, causing all kinds of disgruntled internet postings. My edges are still as shiny as the day I got it.

Yet it IS magnetic which pretty much rules out all of the common 300 series Austenitic stainless steels. However it would be entirely consistent with various 400 series Martensitic stainless steels. Among the 400 series, 410 Stainless is cheap, comes in thin sheets that are readily stamped into shape,is 'stiff', and durable.

So if I were to throw my 2 cents into guessing what it is made of, my money is on 410 Stainless Steel or if not, then some other 400 series Martensitic stainless steel.