The origins of the IMO number
Before 1963 all ships registered at Lloyds of London were allocated a 5-figure identification. This was based on the ship's alphabetical position in the register and therefore could change from year to year. This ceased in 1963-64 when the register allocated a unique 6-figure number, being the Lloyds sequence number (starting at 00001) with addition of a 5 prefix (finishing at 39966). Additions that year began at 540000. Additions (eg new builds) the following year 1964-65 began with 64 and this practice continued until 1970-71. Additionally in year 1969-70 a seventh check digit was added to the 6 figure record.
From 1971-72 numbers were allocated at the time the vessel was ordered not when built. From 1991-92 the first two digits ceased to indicate date of build or order. The organisation IMO (International Maritime Organization) adopted the 7-digit identity on 19 November 1987. Use of the number only became mandatory with (1994)SOLAS effective from 1 January 1996. IMO numbers began to be issued on behalf of IMO from 1998-99.
Go to this site's About page to validate an IMO number.
By the way the IMO number is allocated to the hull of a newbuild and specifically the engine-room area. Therefore, the IMO number is unchanged if a ship is rebuilt on the same hull - but were it sliced into two or more parts to create new vessels the newbuilds would be allocated new numbers. In general display of the IMO number applies to seagoing passenger and cargo ships of over 100 gross tons. Warships and non-powered vessels are exempt, but the categories are ever-changing.
For the purposes of SOLAS the following ships from the database ceased operations prior to introduction of the 7-digit number in 1969 although some survived beyond that date. Ships built after 1969 appear on the list because they are under 100gt, are otherwise exempted (or I have yet to trace the IMO number!)