Apple provides a mechanism that allows the physical Ethernet address of a card or system to be different than the physical Ethernet address built in to the Ethernet card or Macintosh logic board.
The "aa 00 04" is registered for use by DECnet nodes. TSSnet from Thursby Software Systems changes the physical Ethernet address by adding an 'eadr' resource to the System file. Inside this resource is 6 bytes that make the physical address that is to be used.
If the customer is not using DECNet software and wants to change back to the real physical Ethernet address, they can remove the 'eadr' resource from the System file or delete the System Folder and reinstall.