A Naval Registry Identification Number is a unique identifier that the U.S. Navy used for privately owned and naval vessels in the first half of the 20th century. During World War I, in 1916, the U.S. Navy began a registry of privately owned pleasure craft and yachts that were available for patrol service in the event the United States was drawn into the conflict. "Section Patrol" ("SP") numbers were assigned in a series beginning with SP-1 and ultimately extending to well over 4,000. As the registration process continued, other types of ships and craft were included for which the "Section Patrol" designation was clearly inappropriate, and these were generally given "Identification" ("ID") numbers in the same series as the "SP"s. In addition, some vessels that were numbered with an "SP" pr