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When did the United States first issue regular airmail stamps?

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philately Vintage stamp-collecting book.

The United States first issued regular airmail stamps in 1918.1

While Italy conducted experimental airmail issues in 1917, the 1918 U.S. release marked the first regular issue of such stamps. Within the hobby of philately, these early airmail stamps are notable for their historical significance; for example, a famous printing error occurred during this 1918 issue, resulting in a 24-cent stamp that featured an airplane pictured upside down.1