Hi Brett, I agree with Alan and think these were plastic, based on existing cars. And I think the 1955 parts book has just mistranslated into English. Here's the full page of what you describe above, including the original German:Brett Johnson wrote: ↑Tue Feb 14, 2023 7:44 pm Regrettably, the parts catalogs can be incomplete (and incorrect!).
<snip>
Item #8
356.57.355 Plastic window as previously described
356.51.861 "Rear window frame (Wood) -- Convertibles 5015--60389" 5015 was the first Reutter cabriolet. 5001 was the first Gläser cabriolet which didn't have replacement parts
356.51.862 "Rear window frame (plywood) -- Convertibles 5015--60389"
356.57.351 "Rear window glass -- Convertibles 5015--60389"
356.57.353 "Weatherstrip for rear window Convertibles 5015--60389"
It only uses "glass" in the English translation. But this was written by Germans, and in German Glas does not necessarily mean that it is made of glass, only that it is either glass or hard plastic. Note too elsewhere in both the 1953 and 1955 parts book, they describe the blinker lenses as "glass" in both German and English, but all the SWF lenses were plastic. Same with the chintzy interior light lens - called glass in both German and English in the parts book, though clarified there as "Cellon", which was a brand name for a cellulose acetate made by Deutsche Celluloid Fabrik.
Conradt may have well called the rear window "glas" in the original German, but again, that means only that it was hard plastic or glass. And he may have been cuing off the parts book as well, though I believe he had a 1953 cabriolet. Who knows.
There are some existing 1952 cars with their original tops that could settle this question. I'll reach out and see if they are glass or plastic. But I do think this is a mistranslation error on all counts.