It was even wilder than that – Chattahoochee Plantation, established in 1968, was only 10ft wide by 30 miles long, the entire length of the border between the two counties. It had no city government so it didn’t have any laws.
But, GA law doesn’t allow annexation across city borders (Atlanta would have to annex the unincorporated parts of southern Cobb to build MARTA at the time), so this 10ft wide “city” stopped the expansion.
The charter for the “city” was quietly revoked in 1995.