SPECIMENS

The purpose of SPECIMEN overprints is to remove postal validity from a stamp, but to retain it as an example in official records.
Some were kept by the printer as a record of true intended colour, to ensure future printings were colour accurate. Eventually archives were "leaked", or sold off when a major printer (e.g. De La Rue, or American Bank Note Co) ceased trading.
Others went to archives of the Post Office or (for revenue stamps) other official institutions, to show the range of genuine postally valid stamps. When the UPU was established, member countries were asked to send a set of all current stamps to each and every member country. Over the years, some records were sold off.
Mainly the word "SPECIMEN" was used. In some cases the word "CANCELLED" was used instead. The majority of SPECIMEN cancels are printed by typography. Sometimes handstamps were used for convenience where stamps were availabe only in small quantity, for example single sets supplied to UPU countries, where the overprint was applied on arrival. Perfin SPECIMENs can also be found.
In limited cases SPECIMEN stamps were presented to prominent officials or dignitaries as a favour for past services.
Collector interest in SPECIMEN stamps is growing. Available numbers are mostly infinitely smaller than for the regular issued stamps, therefore there is a rarity factor. However, for very high face value stamps with eye-watering catalogue value, SPECIMEN examples can still be found much cheaper than the issued stamp.
Collectors also seek SPECIMEN stamps as a true record of the original stamp, which has usually been well preserved in official archives.
Some Post Offices sold redundant stocks to the public to raise funds, overprinting them SPECIMEN in order to avoid upsetting collectors holding rare examples of the original issued stamps. Those of Australian States come into this catagory. In more modern times, a few printers or Post Offices sold large quantities of so-called SPECIMEN stamps simply to increase sales to collectors. The much lower market price of these late arrivals reflects their status.
Commonwealth collectors who reach an "almost complete" and "budget limitations" stage have been turning to SPECIMEN stamps to fill a gap for an otherwise unaffordable rarity, or simply to add an interesting companion page to a well-developed section of their collection.
In general the current trend is that SPECIMEN stamps are in increasing demand.
Further reading : For GB : "Specimen Stamps and Stationery of Great Britain" by Samuel & Huggins. For Commonwealth : "SG Commonwealth 1840-1970". For UPU countries : "UPU Specimen Stamps" by James Bendon.