According to the Filahome Stamp Collecting Encyclopaedia, firms and governmental departments sometimes punched holes in their stock of stamps to prevent theft. These holes can form letters or an image.
"At first the name of the firm was printed on the stamps. Later, after the invention by the Englishman John Sloper of a machine to punch holes in stamps, mainly punched holes were applied for this purpose."
"Till 1950 perfins were regarded as worthless by serious stamp collectors. The opinion was that the holes made by firms damaged the stamps. Nowadays these stamps are considered to be interesting additions to a stamp collection."
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