MacBook Air serial number

I am seeking to find a Macbook Air serial number from within the backup files created by the Mountain Lion operating system and stored on an external hard drive. Exactly which file do I need to be looking at? I made the presumption that the backup created by time machine would store the uuid to ensure that the physical MAC address (7 layer OSI model) would be the one that received the backed up files of the MacBook Air. Perhaps in Extended Firmware Interface (EFI) folder although I did see a file mentioned; in a post elsewhere which was com.apple.TimeMachine.MachineID.plist. The same post also said that a sparse bundle created by time machine contained the hardware UUID (effectively the serial number?).