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An unissued stamp or postal item is a type that has gone through
the production process, but has never been made available to postal
users. Reasons for non-issuance can vary from change of government,
to change of currency, to errors found at the last minute, or may
simply be due to the lack of need for the stamp at that time.
While in theory unissued stamps would be of little interest because no collector could ever have any, there are several legitimate and illegitimate for them to find their way into collections.
The most common route is for the unissued type to be overprinted; a sensible solution, as overprinting can be done more quickly and cheaply than designing a new stamp. It can then happen that some of the unissued types did not get the overprint, in which case they are classed as overprint-missing errors.
Occasionally stamps are withdrawn in the time between their distribution to post offices and the official first day of issue, but that a clerk in some office sold a few anyway.
Sometimes unissued types are sold directly to the stamp market, either openly, as adhesive labels of little value, or clandestinely, for personal profit.
In StampData, unissued types have all the properties of regular stamps, except that the date of issue is always 0. They need to be created for any type that is overprinted on them; the "Add base stamp" is a useful way to prefill the unissued base stamp correctly.
It also works to create and edit a stamp type normally, then declare it as unissued by adding "yes" to the Unissued field on the stamp's page.