Interesting to see metric dimensions on a wartime A.V.ROE & Co Ltd (!) drawing; I thought they were AVRO long before drawings for the Lancaster (and Manchester!) would have been drawn.

Also interesting that ‘Perspex’ was a trade name of ICI (who ‘invented’ PMMA) and that ‘Plexiglas’ is a trade name of German, and not American, origin (as I had previously thought):

https://en.m.wikipedia.org/wiki/Poly(methyl_methacrylate)