Hey JJZ,
There was a type of glass developed just prior to WWII for use in the airline industry. I think Armoured-Plate Glass was a brand name originally. This type of glass was developed for use in high altitude aircraft initially, but after a short period the glass was used as a general purpose type in commercial airliners of all types, as well as for many non-aerospace purposes.
The glass used a non-standard (for the time) 'alloy' and was tempered to resist shattering, and if it did shatter it should break into pieces that were less likely to cause injury to people. When the new type of glass was combined with detailed engineering analysis of structures and forces, it became a type of 'blow-out' safety glass where the pressure would cause the framing to fail before the glass did, and the intact window would fly out at very low velocity. The glass was used as high altitude airliner windows, and as inspection windows for industrial purposes, just to name two uses that I can think of off the top of my head. Possibly it was used in combat aircraft also??
Eventually this type of glass (or a more developed version of it) became what is generically called high strength tempered safety glass in today's terminology, although the term armoured-plate glass is sometimes still used. This glass should not be confused with bullet-proof/-resistant glass which is usually multiple layers of some form of high strength glass laminated with some form of plastic.
If you want to know for sure whether it is some form of armored-plate glass, and are willing to sacrifice one of the windows, first get a pair of safety goggles!! Then, and only then, place the window on top of something soft (like a foam cushion) in a cardboard box and place a thin layer of something soft over it (a couple sheets of paper will do) so that any pieces will not fly all over. Then use something (a wooden baseball bat would be best, but an aluminum one would work also, a fairly large faced steel hammer if bats are not available) to strike it with a fair amount of force. If the window breaks into large pieces with few sharp edges and almost no fine sharp splinters of any size, it is probably some form of armored-plate glass. If it 'pebbles', again with almost no fine sharp splinters, it is probably some form of early safety glass.
Note, some of this type of glass was very strong (upto about 4x the same thickness of standard glass), particularly if it was designed to leave the frame in one piece rather than shatter, so you may have to strike it with significant force. The amount of force necessary may put a flat on an aluminum bat, or splinter the wooden bat. It is OK to start off light though, since you can just hit it harder and harder until you reach its strength limit. Above all though, keep it safe, for you and anyone else nearby!!