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Does anybody know what material that the original HP-41, 71B, etc. overlays are made of?
Lexan (polycarbonate). You can see any number of companies that make lexan overlays with a web search:

Our company made a product from 1995-2008 or so that used a lexan overlay because it allowed annunciators to be to be totally invisible until lights came on behind them, and then they'd pop out of the black background.
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