I always wondered why my friend's rear boxsides never had rust on them on her 01 crew cab 4 door F150. I was surprised to find lately that they are made of fiberglass! Is it just the crewcab model that had these in fiberglass?

 

It's called a flairside/stepside bed.

You can get them in reg cabs too.

 

But it hasn't got that kind of styling. There is no "step" to make it a stepside. In the old days they were either flareside as you mentioned or the wide straight smooth side called styleside. This like a styleside. Just like a regular metal wide boxside only fiberglass.

 

The Stepside and Supercrew had fiberglass bed sides. The Stylesides were steel.