Page 343 of "The 1967-1968 Camaro Reference Book: From "A" to "Z"..." (1994) from John R. Hooper has a breakdown of the date-coding scheme for convertible plastic windows: Day (1-31), Month (A = Jan through L = Dec), Year (67 = 1967, 68 = 1968, 69 = 1969).
Based on Reply #3 by James (firstgenaddict), the plastic window on the 26k survivor vert was "1H9," not "1H69." So, I would guess that Hooper's book should have only listed the last digit of the model year instead of the last two digits. Hooper's book would therefore decode "1H9" as "1 August 1969." The "3H7" code on the plastic window of Pacecar's 67 Pace Car vert would be "3 August 1967."
BTW, Al Knoch Interiors (
https://alknochinteriors.com) sells date-coded convertible plastic windows...I ordered one from them for my 68 vert a couple of years ago; however, the last time I looked at the Al Knoch web site, date-coded plastic windows weren't advertised. If I recall correctly, you have to phone in your order for a date-coded plastic window. There was an upcharge to do the date-coding.
Bernie