This may seem a little left field but, had anyone considered the posibilty that they were cut from acrylic tubes? More specifically large tube/drum lamp shades that were common in large buildings in the late 70s. It could account for the difficulty in finding the correct colour in cast/extruded sheets today. Lamp shade polymers would be dyed according to the designers taste and smaller runs of items wouldn't be too problematic (hence "designer" lamps being so expensive) Something cut from a tube shape would already have a nice perfect curve on it too. Again this is based on nothing but a hunch, Ive never seen a screen used bobba helmet, the curve may be nothing like what I'm imagining. I think the best course of action would be to try to identify the actual material in the screen used helmet (acrylic/polycarbonate etc) that will narrow it down. I think back then the recipie for polycarbonate was exclusive property of kodak, perhaps someone there may recognise it. It's a long shot..