other details from falling into enemy hands. Similar to this is the "censored postmark", overprinted with a black obliteration of the time and place of mailing, for similar reasons. The Pony Express used a variety of different postmarks on the mail it carried across the Western United States. There are only 250 known examples of surviving Pony Express mail/postmarks in various collections today bearing one of more than a dozen different types of postmarks. Hawai'i Post, a discontinued personal delivery service, once had a surfboard mail postmark, for covers that traveled by surfboard. A colour postmark is on the United States Postal Service-issued collectible envelope commemorating the 2004 inauguration of George W. Bush. While postmarks are applied almost universally by or under the authority of the official postal department, service, or authority in the United States it is possible to receive "a permit to apply your own postmark", called a Mailer's Postmark Permit, and under certain conditions specified by the private express statutes in the United States, a privately carried letter may be cancelled with a private postmark. Unofficial entities that issue artistamps may use postmark-like markings as well. Marcophily is the study of postmarks and there are many published work on postmarks covering the topic from before 1900, such as the fancy cancels, until the present day. These include the so-called fancy cancels of the United States to modern machine postmarks. Fewer postmarks are used now than previously, with the advent of meter labels, some types of computer vended postage, and computerized postage that people can print from their own personal computers (called "PC postage" in the United States, these services have been offered by such companies as Stamps.com and Neopost). These indicia are not always postmarked by the post office but if put into the mailstream later than the date listed on them, they are postmarked about 50% of the time. Because of this, it is a bad idea to try to use the date on your postage as a postmark. An official example relating a numismatic item to postmarks occurred on April 13, 1976 when the U.S. issued a new two-dollar bill. People could buy the bills at face value, add a first class stamp (at the time 13 cents), and have the combination postmarked to show they were the first day of issue. Large numbers of these were produced and they remain common. Ink colour When