Well I told a porky!
I measured carefully my "?L41" and "Triplex" marked glass and it is exactly 4.60 mm thick (not 4.5) I assume this will be an original British glass carefully cut out with an axe!(by the look of it)
Both layers of glass are the same thickness. I remembered it was thinner than normal auto glass because I asked a mate (in the glass game) to cut me some. He told me that I couldn't buy that thickness here. He did however cut me some screens from glass salvaged from an early British built car (Morris Minor or some such)
The Australian pattern carriers use thicker glass.