Pre 1972/3 the main supplier of glass was Triplex after which mainly Sicursiv. Apparently the first digit under the name is the year 2=72 and so on.
The picture shown is one of the quarter lights the other was 1974 so don’t think the 1972 is original as the car is 1975.
The ARVA glass is on the windscreen which i am certain is not original. It is an American made to BS standards and an after market replacement.
I guess if the windscreen gets broken it is cheaper and easier to replace the whole windscreen in it’s frame – which might be the reason why it is there.
Guess there’s not many 40+ year old cars that still have all their original parts (new or old)!!
If you have triplex glass then date codes can be found on this website at http://www.bobine.nl/turner/01-general-information/triplex-glass-manufacturing-date-codes/