Here is some feedback from John who has been researching this issue for some time:

...What we need is some conclusive evidence
(first hand knowledge, internal memo, bulletin, etc) that explains why this image was applied and what factor determined which vehicles at Lordstown during the period Aug. '67 -thru- approx. Mar. '68 received this special glass. So far, none of the research has been able to identify the selection criteria. Even the Fisher Body Broadcast Copy contains no codes for selecting the 'Bird' glass. It's almost as if they were just mixed in with regular glass and installed according to Broadcast based on a Clear or Tinted build order.

We know for sure that it wasn't:

1) A special option (nothing on the Order Form).
2) Connected to any specific option, style or engine combo.
3) Exclusive to just early production Firebirds as several have been found
on cars produced as late as March (some say even June) of '68.
4) Not used on '67 or '69 Firebirds or Van Nuys cars.
5) Unlikely that any cars ever left the plant with the 'Bird' image on only one side (though some claim this to be the case).

I too have spoke with former assy plant workers from Lordstown. They recall this special glass but have no recollection of a selection process
to determine which cars received it.

It will remain a mystery! - Brett - 67 Drop Top 455!