Specimen : Stamps, Post Cards, Postal Envelopes etc.
A specimen stamp is a postage stamp or postal stationery indicia sent to postmasters and postal administrations so that they are able to identify valid stamps and to avoid forgeries. They are samples of new stamps, distributed to the various countries through the UPU for purposes of identification and record or to the philatelic press and trade for publicity purposes.
The usual method of invalidating the stamps is either overprinting in ink or perforating the word Specimen.  
The use of specimen overprints is not restricted to postage stamps. It has also been used on revenue stamps, post cards, postal envelopes, Inland letters and other postal stationery, including International Reply Coupons. Such sample stamps and other philately items, overprinted with the words “SPECIMEN”, are not valid for postage.
As many specimen stamps are worth more than the originals, they have often been forged.