Both Chevelle SS and Camaro BB & Z/28 had the standard chambered exhaust system changed to regular duals on November 25th, '68. After that, the system became available as a separate regular production option NC-8.
Hmmmm, I had a '69 Chevelle SS L35/ 350 hp., 4 spd., 4.11 POSI with Chambered Exhaust. Just now finding out how rare that was. I remember the local Police was always hasselling me about it, I just always told them it was factory installed. Of course I should add I always made the most of it.