Hello -- I too have been searching, unsuccessfully, for a reference to "Old Mexico." My great grandfather's WWII Draft Registration card said he was born in "Old Mexico." I have no other clue where he was born, other than this reference. Were you able to confirm whether this was simply the country of mexico, or the portions of the US prior to the Treaty of Guadalupe Hidalgo?
I also can only obtain names of Mexican restaurants when I google the term :)
Thanks for any information, Roxanne