With a magnet you can be sure. Alu and titanium isn't attracted, steel is, stainless 304 very weak, stainless 320 not at all.
I rather doubt it's aluminium, seeing the thickness and sharp bending, aluminium alloy isn't that ductile and quite weak as a thin plate, especially with holes drilled on the corners and mounted there.
I once got an aluminium spacer mounted on the rear axle of a fixed gear, one the dealer recycled from a gear cassette or so, with 3 holes drilled on its circumference. When I left the shop and pushed the pedal, it broke immediately. Just to illustrate. Handlebars do undergo some force, and it's also under leverage.