U Security Machins
U Number Machins were first issued in 2009, and are often referred to as "Security Machins"; this is because they have several features that make it difficult to create forgeries, or re-use used stamps.
Original issues featured U-shaped slits on the face (making it difficult to peel-off unfranked stamps to re-use) and self-adhesive backing (making it impossible to soak them off the envelope to re-use, as self-adhesive stamps dissolve).
Later U Value issues feature an iridescent overprint on the face, with a source code and a year code, making forgeries easier to identify.
It appears that no new U Number Machins will be issued, now being replaced with Barcoded V Values.
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