If we can trust the images in the gallery most of the helmet has in some degree a wavy brow.
The PrePro #1 and PrePro #2 has a slight wave in the right side
[ATTACH]28791[/ATTACH] [ATTACH]28792[/ATTACH]
The PrePro #3 has also a slight wave and the ESB a bigger wave on the left side
[ATTACH]28794[/ATTACH] [ATTACH]28793[/ATTACH]
I don't see it on the ROTJ helmet and I think we can't count on the Sandy (because of the severe damages it has).
This wavy brow thing might be because the way they fixed the visor into the helmet, but maybe not.
The only difference is this: if you want to see the wavy brow, then it's there, if you don't want to see it, then it isn't there.
Sorry for hijacking this thread (too).