They've never been listed as anything but aluminum. However, it would have been simple for them to do a run of brass ones. Maybe they did. There would be no way to tell from looking at it, only by comparing weights.
IIRC on the Hipshot A and B bridges, both the aluminum and brass versions have brass saddles. i.e. the choice of aluminum or brass refers to the machined base. So it wouldn't surprise me if the Supertone also had brass saddles.