If your '76 is anything like my '74, you have an aluminum snap ring around the inside of the window, holding the glass in. Airstream made several different styles of Vista View and Stack window assemblies. Hopefully you have the same ones I do, as they are easily rebuildable. I wrote up a pretty thorough procedure with pictures and all... Let me know if this is the style you have, and if I can be of any help...
http://www.airforums.com/forums/f454...ign-96251.html
-Red, first thing I ever did on my camper...