During the course of looking for replacement seat covers, we discovered that there was a mid-year change in the 1968 Firebirds. The 400 HO is early (November 67).
According to Legendary Auto Interiors, the custom option for seats in early 1968 builds have a madrid grain insert, later year builds have a comfort weave. This led us to hours of searching on forums. To add to our confusion, our HO came with vinyl bucket seats and a rear seat with the comfort weave. We knew the bucket seats were not originals, but had no information on the rear seat. It certainly looks old enough to be original.
Eventually, we found the most comprehensive list of differences at the First Generation Firebird website in their FAQ:
Indeed, the knit weave insert for custom option seats became available mid-year.