In an article in the Technical Manual Jim Mahaffey writes:
.... 7. Weathering. Red rouge on a cloth wheel can do wonders for the plating on the brass bezel, and even for scratches on the plastic crystal.
8. Cracked or smashed crystal (very rare). Use the clock for parts. Assure the owner that they're all just alike, and he won't notice the difference if he buys a new one. ...
So I guess it is some kind of plexiglass.