The mystery of the painted on bird on some 68's glass has been discussed in detail on this site if you search.
Rocky Rotella, in his "
The Definitive Firebird and Trans Am Guide 1967-1969" book offered new insights into the mystery that I have not seen before. He said that the glass bird was planned for all '68s but when a federal vehicle standard prompted them to discontinue the baked on ceramic paint bird because it might obstruct the view in the side mirror . He states that most are on 11/67 and 12/67 Birds but showed up sporadically throughout the rest of the production year as they depleted inventory.
That pretty much matches the research by the Camaro Research Group statement about their Astro-Ventilization side windows: "The Astro-Ventilation logo was dropped from the 1968 door glass about Nov or Dec '67 production. However, one pane of apparently original May '68 door glass with the Astro-Ventilation logo has been observed in a 05C build. The mating glass on the opposite side of the vehicle is Sep '67 Astro-Ventilation glass. We believe that the May '68 pane had the Astro-Ventilation logo added by LOF to create a matching set, presumably to use up left-overpanes. At least two other late '68 cars have been observed with late 1967 Astro-Vent glass."