Strictly speaking, these SP/ID registry numbers were not U.S. Navy "hull numbers", which would not be formally adopted until mid-1920. Many of the ships and craft so numbered had no Navy service, while others that were acquired and employed by the Navy received no numbers. However, like hull numbers, the SP/ID numbers were used for record-keeping purposes and were often painted on the exterior of vessels (especially patrol types) to facilitate identification. They can therefore be considered precursors of the later hull number system and are included here for that reason.