Jim is correct on this and there is a copy of that dot memo from 67 that I have seen floating around on the web somewhere. The only thing I would add is that some cars came through with only bird glass on one side. I have personally verified this on 2 original unrestored cars where all the glass was date coded the same. Both cars had the bird on the passengers side leading me to conjecture that because there was no right hand mirror on these 2 cars to obstruct they felt safer depleting that unused inventory on some of the lines this way. But it also could have just as well been because they were out of plain right hand glass at the time and did not want to slow down production. Food for thought. I have owned 4 sets of bird glass over the years and to me there is nothing sexier than factory green tinted bird glass.