It is amazing what you can find with Google. How about this article titled:
Ageing Triumph GT6's using the window glass !
Most classic cars can be dated by their registration letter or registration document. But what about cars which have personalised, foreign or re-registered number plates, or were registered late by the original dealer ?
The most accurate method is, of course, to contact the BHIMT who may be able to supply such details as production date, date registered etc. However a rough guide to a cars age can be found using the manufacturing codes on the windscreen and door glass.
On the glass underneath LAMINATED, TOUGHENED, ZEBRAZONE or WIDE ZONE there may be a dot. If this dot appears under the first letter, then the glass was manufactured (blown ?) in 1961, 1971 or 1981 (and I think it's safe to say that we can approximate the car's age to within ten years !). Similarly, if the dot appears under the second letter, the glass was made in 62, 72 or 82 and so on, with the dot appearing under any of the nine letters (or eight + space). If there is no dot, then the date is 1960, 70 or 80.
The month of manufacture can be found on TRIPLEX glass. In this case only the first and last pairs of letters are used, as shown :Triplex
Now, this is only a rough guide as mentioned, as it relies on a strict factory stock rotation, so expect a few anomalies. Furthermore, it is a good idea to use the door glass, as there is a fair chance that the front windscreen will not be the original.
This post originally appeared on http://www.fugazi.co.uk/GT6/GT6.html