I hope it wasn't something that I said.
And yet the thread lives on. I guess people care about the underbody. Anybody have a guess what a 'diffuser' looks like?
I always thought old VW buses came with a bellypan, at least the ones with double doors or a sunroof opening, to stiffen the body. But I went looking for an illustration and I was surprised by how little it covers:
There is also a removable pan that covers the front between the rails. I'm going to compare and contrast with this:
Which picture may be the reason the O.P. started the thread in the first place. It is pretty.