As usual, Glen hit the nail on the head. Also, in general, the horn will be lighter coloured the younger it is, i.e. nearer the centre of the horn.
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Really interesting and great photo. Do you happen to know when they go about cutting the horn, is it bisected right down the middle and then flattened, or something like cut the the centre and then flattened out in one piece (the way one does with birch bark, for instance)?