You can get an idea of what the market looked like here:
http://www.davidgleason.com/Archive Market Ranks/Charlotte.pdf
It was a two-horse race between WBT and WAYS from '75 to '79 - with WSOC-FM reaching the upper echelon by '79.
Unfortunately these numbers are averages of the Spring and Fall books from each year, and I am looking for the specific 12+ numbers from each book from this period and earlier.