To see how you can determine what mainboard version look at the S/N of your device. The second and third digit in S/N tells you about the mainboard version. If the second digit is X you have an X board if second/third is AX you have an AX board.
The downloads below are all for the same OS version but for different hardware configurations. Pick the one for your device by looking at your S/N. For example X4 is for an S/N starting with I600X4…