The 'stiff cardboard' is likely made of layers of paper, impregnated with phenolic resins. Not a whole lot different from the stuff Formica counter tops are made of.
Here are a few more pics, of similar breakers:
First, from Norcal, is the breaker and it's packaging:
I recently came across these on a job. The panel looks like this, all closed up:
Notice how the handles seem to be 'all over the place?' Every breaker in that pic is "on." A look inside reveals what is 'strange' here:
There is but a single column of breakers. You decide which buss is used by flipping the breaker over.