I picked up a pretty old looking Birdie at a pro shop in Spring, TX last weekend for $5. The stamp is like 95% worn off, and all there is on the back is an imprint in a circle that says Innova Champion Disc Inc., Ontario, CA 91761 and has a patent number on it. I'll be honest, I have no idea what type of plastic this thing is, but I know it feels good to putt with. Any idea how to determine what the plastic is?
Edit: Just called my wife to take a look at it, she says she thinks the stamp was a "foil" stamp, but it's worn to the point that it's really hard to tell.