I could be wrong, but if you are talking about the rear panels in the A/S I suspect they had them made up specially......
I have broken the n/s rear in Gunther and have been looking for a replacement for a while and have struggled, defo not the same as a factory window and different to some of the other conversions as well.
My assumption therefore was that as the A/S was a converted panel van they used their own windows or ones that were available in the aftermarket at the time.
Sure Mick will put me straight though if I'm wrong
Cheers,
MM