I don't believe it make's any diff on the material, but the thing that I have found is the grooves in the facing, one will have grooves straight across the face and the other will look like a waffle, the only other thing to watch is the thickness of the fiber plate, when I got my master overhaul kit from Mancini the fiber disc's were all the same blue plate material.

Light travels faster than the speed of sound,,,this is why some people seem bright untill you hear them speak.