I've never seen any documentation on rear disks being available in the 1978 model year, and have allways understood that it was not. I'd love to see some documentation on that, either from an original owner with a build sheet or some other verifiable source. I disagree with the statement that some cars had them in 1977, unless you are talking about the 8 Trans Ams that Herb Adams built for Pontiac that led the Fire Am project. Those were not production cars.
It certainly was not normal for 1978 WS6's. My car is a 1978 WS6, built May 1978 in Van Nuys, and it has rear drums.
I think that there probably were quite a few tuners or owners who converted 1978s to rear disks with factory parts, so saying you bought a used 1978 with rear disks doesn't count.
By the way, WS6's also had small braces from the rear of the lower control arms to the subframe.