I think there's some confusion going on here. The "earth" pin on the duck head is black plastic. The metal pin on the actual PSU is an earth pin, but the duck

I think there's some confusion going on here. The "earth" pin on the duck head is black plastic. The metal pin on the actual PSU is an earth pin, but the duck head isolates it since the sleeve part that fits around it is all plastic. The extension cord, on the other hand, has metal in the sleeve and has a proper metallic ground pin on the mains plug.