The Ethernet interfaces being swapped is something that happens frequently in the transition to ath79. I suppose there's a technical explanation for that but I'm not familiar with how that works exactly. The question is not how to replicate the Backfire behaviour, but to match the addresses to the stock firmware.
The MAC addresses are extracted from locations on the flash defined in the DTS; the Nanostation's DTS points (amongst others) to the XM DTSI which defines this:
As you can see both MACs are extracted from the ART partition. It looks like that's the wrong match for the Nanostation M2 then, unless that's how the stock firmware behaves. Only someone who can run the stock firmware on the M2 can provide conclusive evidence on that. I checked the Wikidevi clones but they haven't logged the MAC addresses either...