No, there has been no evidence to support one way or the other how or why which Firebirds received the 68 Bird Glass (which again, is not an etching process but they were printed on the inside of the glass) nor has there been any way to determine which cars received the Glove box door Bird on 67's & 68's?

I know a few have collected build sheet data, PHS documentation and any other type of information in hopes of determining an appearance group, a trim package or some type of option package that may have correlated with the addition of either the Bird Glass or a few other options such as this, but there is nothing to support this nor does it seem to be called out anyplace...

So yes, that IS the only answer at this point that we know of.