I wouldn't call the spots an error but more a printing oops. The phosphorus used in tagging is a powdered substance that requires contiuos mixing and spatter from the machine is not to uncommon. I have a pair of $5 moose that only used the phosphor for a hidden anti counterfeit mark but is literally covered with glow in the dark agent. Shown them to many specialist and no one will confirm them but being in printing years ago I know exatly why it happened.
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