It's based in APX 100. Slightly thicker emulsion with higher Dmax and contrast. Different base (clear). Same spectral response (because it's the same chemical emulsion just coated thicker).
My earlier comment about antihalation layer was a tongue-in-cheek joke about the peanut gallery responses we would hear if they tried to claim that it truly was APX 100. I have no evidence that the antihalation layer is the same or different. It was a joke.
But again, I'll say that I think we can treat it like APX 100 and get "substantially" the same results as if it were the real thing.
I think Thomas's idea of a step wedge is great.