in 1951, to make way for new j and r class engines being ordered under operation phoenix, the remaining d1 and d2 engines were renumbered to the range 561-579 and 580-604 respectively, with d 604 changing to 606. at the time, engines d 573, 578, 579 and 585 were still in service and retained their numbers, leaving gaps at numbers 575, 577, 583, 602 and 605. assuming 585 would have been renumbered to replace d 572 withdrawn that year, the remaining open slots in each group correspond to the number of engines withdrawn in 1951.