Backstamp

A backstamp is a marking applied to the back side of a stamp. It can be:
 * a control marking by the post office. This would be placed on stamps before sale and would be washed away once a used stamp was soaked.
 * a usually small handstamp usually of a name and a city indicating a stamp was sold by the dealer named. A guarantee is implied with that though it is not always a guarantee of genuineness.
 * a usually small handstamp of a design or letters used by an owner to identify stamps in case of theft.
 * an expert mark denoting a stamp is genuine. Be aware that these have been forged and that every name you see on the back of a stamp is not necessarily an expert or even a reliable expert.

See

 * Backprint

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