Recently I came across a used copy of the 12p emerald green N. Ireland Regional Machin (which was issued in sheets with one left phosphor bar) on which the phosphor bar appears much more vivid to the naked eye than usual.
Is this a variety or is there another explanation?
An image is attached which shows three copies: the first (from the left) is most typical of the ones I have seen; the second shows the phosphor bar slightly visible to the naked eye; and the third is the surprise.
