United States. Postmaster General. (1907). Advertisement inviting proposals for carrying mails of U.S. in island of Porto Rico, from July 1, 1908 to June 30, 1912. GPO.
Chicago Style (17th ed.) CitationUnited States. Postmaster General. Advertisement Inviting Proposals for Carrying Mails of U.S. in Island of Porto Rico, from July 1, 1908 to June 30, 1912. [Washington]: GPO, 1907.
MLA (9th ed.) CitationUnited States. Postmaster General. Advertisement Inviting Proposals for Carrying Mails of U.S. in Island of Porto Rico, from July 1, 1908 to June 30, 1912. GPO, 1907.
Warning: These citations may not always be 100% accurate.