It is "particle board" but it's the stuff we make speaker boxes out, 3/4" mdf, it's pretty durable stuff for the most part as long as it doesn't get wet. There are no signs of water damage on it. It could be ldf (low density fiber board), but the grains in it look too small, smaller grains, higher density, stronger board. It's used because you can drill through it without much chance of splitting it and it can be sealed air tight with glue and silicon.
But, mdf and especially ldf are not structural.
Also, we have been trained to roll or walk over 70's out of the truck, so you may have a problem with your claim. Sorry.