Looks like it's possibly a modern addition for the comfort of the occupants, although quite why anyone would make it so upright (and ugly?) is anyone's guess.
Period pics show a variety of screens fitted, but this is the one from Moss's record breaking Mille Miglia win -
Much lower and more aerodynamic looking and, from experience with my Spyder, provides a similar amount of wind in the face protection to a low, full windscreen like you'll find on a Midget, 356 Speedster or early Spitfire.
Still a beautiful car though, perspex garden fence and all.