In a way, that makes more sense, but Doug's photo looked rubbery like PU might be. But I am unaware of any flexible sheets of medium density PU, so... which is it??? Doug and all other sources I've seen so far say it's PU. I think I'll buy a couple to check out anyway and I'm confident that the feel will give it away.

Would be trivial to measure the density and that might clear up any PP vs PU question - but PU comes in so many densities...

Doug?