Kodagraph Transtar TP5. You can fold it, backlight it like a transparency, write on the emulsion with pencil (and erase), and it's red-hot in lith. You can print through the back, and with a strong enough light project it on a wall.
I've been told it was a graphic-arts paper, but the way in which it was marketed (very expensive, only sold in large sheets and rolls), the ambiguity of its purposed uses on the data sheet (which actually contains no data), and the fact that I cannot locate anybody in Kodagraph Ontario where it was produced who has ever heard of it, this tells me it was not produced for the masses.
Always interested to hear from anybody who knows anything about it.