Indeed A-Sedan did get phased out for a while, but as I can tell, A-Sedan was at least running through 1979. I have the 1979 SCCA General Competition Rule book, and pages 105-116 covers A-Sedan. In 1980, the SCCA began lumping the cars together, and even provided a supplemental document to the GCRs entitled "1980 GT Category Specifications." The 1967-69 Chevrolet Camaro is classified as GT-1 in this supplemental document.
But, if someone is interested in the Camaro from the Craigslist ad above, please do more fact checking on my bullet points. I did not further research the material, and only pasted these details as the 2001 owner presented them to me.