Secret mark

A secret mark is a tiny design variation done on purpose. The idea is it is used either as:
 * a way to distinguish printings or printings from different printers.
 * a security feature for postal authorities to detect any postal forgeries.
 * A feature added to distinguish official reprints from original printings.

Secret marks are by definition quite small and will require you to use a magnifier to detect them. Even then, poor printing of earlier stamps can make identification difficult, as is sometimes the case with US Banknote issues.





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