Triplex windows manufactured in the 1950s through the late 1970s can be dated with the "Triplex code" etched into the glass.
Assuming that your car has the original glass, it can be used to fairly accurately date your car. Of course, this method reveals the date of manufacture of the glass itself, not the car or hardtop it is fitted to, but the glass probably didn't sit around very long in the factory before being fitted.
Triplex Safety Glass is now owned by Pilkington, which uses a similar method of dating using dots under letters, but is not described here.
Triplex Window Date Code Decoding
If you know the decade your car was produced, you can find the last digit of the year by looking for two dots in the TRIPLEX logo on the glass.
One dot above T, R, E, or X gives the quarter of the year the glass was manufactured:
T is Jan, Feb, March, R is April, May, June, E is July Aug, Sept, and X stands for Oct, Nov, Dec.
To find the year the glass was manufactured, look at the nine letters in the word TOUGHENED.
One dot below a letter gives the year of the decade:
T is 1, O is 2, U is 3, G is 4, H is 5, E is 6, N is 7, E is 8, D is 9
If there is no dot, the year is a zero.
This code also works if you have a TRIPLEX window with the word LAMINATED instead of TOUGHENED.