Your right Lanc. Perspex appears to have been pretty common on both sides of the pond for allied bombers. A better question for you (and I truly have no clue on this one), is do you know what properties perspex exhibited? Bullet resistance? Less crazing? Shatter resistant?
Is this a Dow Corning invention or one of the Brits? I too had not heard of it until I bought a book entitled "Gunner" that details gun positions on various allied and axis bombers, provides insight into personal gunner observations, hit ratios of various aircraft by type, circular hit probabilities for various turrets and waistgun positions, technical features, etc. Fascinating book if you are even remotely interested. Great photography too.