Just my opinion, I don't claim to be an expert. But I don't believe that the screens sold under part number 700896 looked like that. Although somewhat different than the ones supplied now, they still had (partial) chrome frames and flat glass. Even Piggott mentions "a larger glass area than the type commonly sold today". I don't think he would have said "glass" if they were plastic.
No doubt there were some plexiglass screens made and used, likely the eBay seller is even telling the truth about getting some from a dealer (who likely made them himself or had them made, for his own car). But I don't believe they were ever "factory optional". The works cars had quite a few things that never made it to production (like the plastic side curtains on OVC 276); and both photos show differences to the ones being sold.
That said, it's your car. If you like them, buy & use them. I think I'll pass.
BTW, I've been told that it is illegal to drive on the street with a plastic windshield, even with a car made before all the safety regulations went into effect. Don't know if that's true or not; and maybe you could get away with arguing that it was "factory". But the police in most areas can impound cars that are "unsafe" and it seems like a risk to me.