I think the lensey-bits are a different shape on the clear ones - the coloured patches are much more rectangular. The clear ones also tend to be a lot bigger (although I have seen some big ones with coloured patches).
Don't quote me on any of this - I'm just digging around in some dusty recesses of my brain, but I'm of the opinion that these is a FV design a bit like the French ones - so there are some french ones (of poorer quality) and some FV ones (of superior quality).