According to William M. Senkus of Alphabetilately.com, "Stamp collars were a popular device in the early 20th century in Europe, especially in France (where they were called "porte timbres"). They were most often used for patriotic or charitable messages. One received them in return for a donation to the cause, then used them either alone, or with a postage stamp added."
Senkus writes, "The center, where the stamp could be placed, was sometimes totally blank, but usually bore a simple message either supporting the organization selling them, or explaining their use. The border contained a message to surround the postage stamp and promote the cause."
He goes on to say, "In the US, stamp collars were printed on envelopes (see above), usually as advertising, hence they were called "Advertising Collars."
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