It's the max tow which isn't the same as HDPP. All PB get all of the same base frame/springs and cooling features. I believe it's all the same be it FX4 or longer bed.I believe, this from the greater detail available on the '21's, is that PB is 3 leaf but not the same rate as max tow.
We got under the trucks and pulled part numbersI'll be away from my laptop, and the links, for a week, so... my pretty strong recollection is the published spring rate of PB is not the same as Max tow or HDPP. I'll try to remember to look for it when I'm home again.
Of course I could also be wrong, so...