Maybe it's made from Imperial Legion and Palace Guards
No, I'd guess that it's made from some form of iron or steel, my reasoning being that it seems like a fairly mundane sort of armor, so I'd expect it to be made from mundane materials, and it's quite obviously at the very least metallic. Also, In Morrowind, the heavy plate armor worn by Imperial guards was reffered to as Imperial Steel, thus indicating that it's made from steel. If the stats are somewhat higher than normal steel armor, I'd guess it's either because it uses higher quality raw materials or the refining proccess is better, or the design itself allows it to provide better protection.