Pretty sure its transparant T molding with aluminium inside.
Zaccs have this also, and i think its an Italian thing.
The bronze/gold color is the colorfading of the transparant due to whatever reason.
maybe its the same issue as the old plastics and its the fire retardant that causes the discoloring.
Here a pic of the bottom of my zacc, it has the T molding damaged because i dont have the leg levelers in, you can see the exposed aluminium.
also if you look to the right side next to the damage, it looks bronze/gold, but left looks siverisch: