The Information Security Oversight Office has published an expanded guide explaining how to properly mark classified documents. See “Marking Classified National Security Information” (pdf), October 2007.
A 1972 monograph prepared at the National Archives reviews the history of information control markings on military documents back to the 19th century and traces their development up to World War II. Such markings represent part of the pre-history of today’s national security classification system.
See “Origins of Defense-Information Markings in the Army and Former War Department” (pdf) by Dallas Irvine, National Archives Staff Information Paper, 1972.
A sizable collection of old military regulations relating to protection of confidential information is presented in a series of annexes to the paper (pdf).