The serial number can be found on the bottom sticker on the original packaging. It's displayed in the format of X3C-#####.
It can also be found behind the backplate in the bottom left corner.
The third and final way to retrieve the OP-Z is to:
* Boot the unit in upgrade mode.
* OP-Z will show up in the computer as an external removable usb disk. Open it.
* Then open the folder "systemfolder"
* Open the text file "serial.txt"
* Your OP-Z serial number will be displayed in the first line, next to "unit: "