Since the M1 is big brother of Neo (I just took Neo micro SD card and used it to boot my new M1 without problems), the answer from M1 might be the same as for the Neo. And the answer for M1 is "its a random number":
http://wiki.friendlyarm.com/wiki/index.php/NanoPi_M1#Ethernet_Connection
The NanoPi M1 by default doesn't have a valid MAC address. If the board connects a network successfully it will automatically generates a random MAC in "/etc/network/interfaces.d/eth0". Users can change it to a self-defined valid one
Hermann.