Touring and Pinin Farina used Plexiglas before the war. Touring even promoted it in adv as a light material from aviation.
I think we need to think about 163 in a different way. Almost all racing cars those days were open, without roof. Closed aerodynamic bodies appeared in racing in 1937. Because of the shape the visibility was limited. When we read any article about driving experience of 2900LM we understand that the cockpit was tight, noisy, hot and with very limited visibility. However considered that driver had better comfort because he was protected from any kind of weather.
I just can assume it was first attempt to make a drivers life more comfortable
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I see your point, and I might indeed agree. On the drawings in Fusi, there's no explicit difference etween glazed surfaces and metal panels, especially on the roof. Moreover, Biema strangely added a small quater-window behind the doors, which is NOT on Fusi's drawing (attributed to Colombo in Ruocco's text).
I would indeed interpretate the factory drawings without these small windows, and also without roof windows: no need for them, and look at the front view (Fusi p.412): no glazed roof. That confirms that the upper view just shows metal panes on the roof. Same with the cutaway side view p.414: the doors semme toi extend slightly higher, no trace of glazing on the roof neither behind the door.
I would instead believe that the four rear windows over the engine are clear. Not sure about a firewall, and if there was one, it could have had a window for rear view in its top.
BTW, in my own research at the archive, I found definitely mention of a 512 engine modified for fitting into the 163 (along with the 512 gearbox) still in stock in December 1945, not of an S10-derived V12. Also metioned in 1941 is the then ongoing construction of two sets of partts for ungrading 162 engines into 163s, with the planned contruction of 4 reinforced crankcases. Pencil writing, May 1941, indicates a plan for 3 cars plus two further engines.