brass, more than likely these will be repainted and set back out again looking like something for a charitable org.
there are so many of these things stolen and then repurposed for a for profit org (stealing) that i take alll of my stuff to goodwill, and never to a drop box.
hopefully you can locate your boxes, try putting some type of identifyer on them, maybe spray brake fluid on the back with your company logo, will rust and look like poo but they cant repaint over them.