Censorship can be a quirky business.

Unfold a 1944 map of San Diego County by the Automobile Club of Southern California and look for Lindbergh Field, the port, Point Lomas Naval Training Center or Marine Corps Depot. You wont find them. Nor can you locate Fort Rosecrans, Camp Kearny, Navy Hospital and the Ship Repair Base, Camp Pendleton, or the 29 other military installations in the region nicknamed Defense City No. 1 during World War II.

Now, unfold a second 1944 map of San Diego County by the Auto Club. It has the same cover and legend box as the first, but every airfield, military base and pier is clearly marked and indexed.

Youve stumbled across a little-known relic of World War II on the American home front. In an era when Google Earth and GPS offer instant mapping worldwide, the idea of map censorship in the United States comes off as ludicrous. Yet, 72 years ago, following the December 7, 1941, Japanese attack on Pearl Harbor, map masking as the practice was called became official wartime doctrine for road maps and others issued in the U.S. for non-military use.

Under a voluntary code distributed by the Office of Censorship in early 1942, map-makers along with journalists and other purveyors of information were asked to remove details that would disclose locations of ammunition dumps or other restricted Army or Navy areas along with the locations of forts and other fortifications. The code was self-policing: media were to ask themselves, Is this information that I would like to have if I were the enemy? and act accordingly, says historian Michael Sweeney, whose book Secrets of Victory examines the bureau.

For the Auto Club, which worked closely with West Coast defense authorities to craft maps for the military, its cartographers were often able to print two sets of maps: fully detailed renderings of the state, counties, and cities for the armed forces; and censored editions for civilians thus the contrasting 1944 San Diego County versions.

For other map-makers, especially those producing consumer maps for gasoline brands to distribute, masking proved inconsistent, quirky, and even darkly humorous.

Most U.S. oil company road maps were drawn by H.M. Gousha of Chicago/San Jose, by Rand McNally of Chicago, or by General Drafting of New York. These often colorful maps had exploded in popularity during the 1930s with the growth in motoring. Americas December 1941 entry into the war, however, generated wartime paper shortages, gasoline rationing, and a crimped market for tourism. The companies nevertheless issued 1942 maps for most states and cities, as they had already begun production, but not until 1946 did updated maps again appear broadly. (Maps from 1942 were occasionally reprinted during later war years.)

So, while the Auto Club could derive ongoing civilian issues from its military versions, the major map companies had to choose what to eliminate on each of their prewar issues within a one-time span of a few months. Depending on the cartographer, their wartime maps of the same area differed as to which, if any, airfields, ports, dams, oil fields, military bases, and related facilities disappeared. Even among maps drawn by the same company, the level of masking varied based on the gas-brand label.

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Censorship can be a quirky business.