Is there not a manufacturer's label anywhere on the door?

It is usually in an area that we refer to as the "end cap." It will be around on the side where the track is and could be on one or all of the panels.

i'll check the end caps. good idea!
all i have found, so far, were scribbled measurements on the top and bottom sides of the panels(in between the "folds").i can;t rememebr if i did look at the ends, though, so it's worth another glance. thanks for the idea!
also, i found it may fall under the category of COMPOSITE.

...some kind of masonite

A composite would be right. Essentially glue and wood fibers.

...if cross section is cut, it looks like a honeycomb cardboard inside.

Hollow core. The hollow part reduces weight and the amount of material needed to make the door. The honeycomb adds strength.