I'm also into restoring old houses, but plaster is not common here in Norway so I have no experience with it.
When working on the windows in a house from 1910 I got hold of some other old windows with the correct type of glass. A company was throwing them away so I got them for free. The leftover glasses was traded for other parts we needed for the house. Unfortunately the house had to be sold, but I stay in contact with one of the previous owners who now wants to buy another house for restoration.
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Quoted from DCP:Our cupola seems to have vanished sometime in the late 1940s to early 1950s, probably because it was needing repairs and looked "old-fashioned" by then.
Same thing happened to many buildings here, like this one from 1897:
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