67 Vent window issue...

[HIGHWAY BY THE SEA]

When I bought my 67 GMC I noticed that BOTH vent window frames were intact, but the windows themselves were plexiglass. I figured that over the years the former owner(s) had somehow broken them and replaced them with plexiglass.
Well, I bought a very nice used set of 67 vent windows, and when they arrived one of them had separated from its frame even though they were packaged very securely. This got me to wondering if there is an issue with whatever holds these 67 glass pieces in their frames somehow deteriorating over the years and the glass falling out. Anyone else have this issue? And what type of adhesive should be used to repair it?
I sure am glad this happened in postal transit to my house instead of after I installed them!