It's a trade name for clear vinyl.

http://www.boatdocs1.com/Clear-Vinyl-Windows.html

On 30-Jul-06, at 10:00 PM, Automatic digest processor wrote:

> Date:    Sun, 30 Jul 2006 18:47:35 -0500
> From:    John Rodgers <inua@CHARTER.NET>
> Subject: Isenglas Windows [NVC]
>
> Can anyone tell me just what isenglas is, and where it has been
> used in
> the auto world. I seem to remember something about Henry Ford, the
> Model
> T, and isenglas windows or headlights or lamp covers or something. It
> has been a long time. But it let light through. The few references
> I hav
> found so far call it "plastic". I don't think that is right.
>
> Anyway, I would appreciate any feedback.
>