Both were rebuilt in 1942 from the LMS Jubilee Class engines—5736 Phoenix in April 1942 and 5735 Comet a month later. They were the second and third examples of the 2A-boilered locomotives, following on from the 1935 pioneer rebuild 6170 British Legion. Both Jubilees had been built in 1936 at Crewe Works, and therefore their original type 3A boilers were not life expired and hence were refurbished and reused on different engines rather than being scrapped. They were given the power classification 6P.
These two one-offs were have said to be a direct upgrade in performance in both power and steaming abilities from the non-rebuilt Jubilees, and similar in performance to the Rebuilt Patriot Class, however they were 3 tons heavier than the non-rebuilds, thus limiting their route availability.
From 1943, it was decided not to repeat the conversion on the remaining 189 members of the Jubilee Class. Rather it was decided to rebuild members of the LMS Royal Scot Class and LMS Patriot Class into the LMS Rebuilt Royal Scot Class and the LMS Rebuilt Patriot Class.
