in 1929 the d series was further segregated into d, d and d taking slots 500-645, 700-799 and 638-699 respectively. the first of the latter was d 542 to d 685 in 1929, followed by further examples of the d upgrade completed in 1930 to give the number range 675-689 and this was further extended to 670-699 by the end of 1932. later conversions between 1933 and 1946 counted down from 669 to 607 in 1946, and finally 604 ex d 717 entering service in 1947. it is not clear which, if any, engines were intended to take the slots of d 605 or 606. otherwise, the d group was reclassed as either d or d as appropriate, for the most part without renumbering. unlike with other renumbering projects, engines converted to d and renumbered did not have their previous slots immediately filled.