B. Free Franklin Post Office, Old City Philadelphia, PA. The only Colonial-themed post office operated by the United States Postal Service. It is the only active post office in the United States that does not fly the American flag (because there was not yet one in 1775 when Benjamin Franklin was appointed Postmaster General). The postmark &quot;B. Free Franklin&quot; is still used to cancel stamps. The museum on the second floor features displays of postal history and memorabilia.