Just had my top and window replaced this year. It is the one piece plastic, no zipper. The two other tops i had were also one piece plastic without zippers. I see they sell the folding glass rear windows for FGF's but i do not think they were ever factory on an FGF.I went with a KEE auto top and window i purchased from Earlybirds in Toronto,Ontario. http://www.early-birds.com/parts_223.html I believe you can purchase just the plastic window if you wanted to go to a zipperless window.Both my fathers oldsmobiles have the folding glass.As Jim mentioned,the glass will tend to seperate from the vinyl it is attached to.The rails are black, someone probably did that for show/bling purposes.
My window as well, will crease when i fold it back.Best to keep the top up when not driving it,summer or winter as it will keep the top mmaterial stretched.

David