I have owned a 1967
22' safari for almost 40 years. I still take it on the road for both long and short trips.
Tom Nugler is correct when he says the original windows were made of Corning tempered glass, but those windows are not flat. They are curved to fit the profile of the airstream's outer skin. They do not have the aluminum edge on them.
Someone on this forum has stated that someone else was re-manufacturing a few of those windows. Sorry, I don't remember who that was or who was doing the re-manufacturing. Perhaps someone else will remember and be able to provide you with that person and you can contact him direct.
You can have the plastic windows made for replacements, but you need to provide them with the exact curvature needed. It's a pain, but not impossible.
Good luck! -- Don