70 members of this class were rebuilt by the LMS and its successor British Railways (BR) from LMS Royal Scot Class engines by the replacement of their life expired parallel boilers with a type 2A boiler over the period 1943–1955.

(The class sometimes is numbered 71 as it included a prototype 1935 rebuild LMS (4)6170 British Legion, but sufficient technical differences existed which dictate that it is treated separately).

In the 1948 Locomotive Exchange Trials, carried out by the recently nationalised British Railways, involved 46154 The Hussar and 46162 Queens Westminster Rifleman.

Apparently a rebuilt 'Royal Scot' proved capable of matching the maximum performance of larger Pacifics.

Of the 70 engines to be rebuilt only 2 members of the class have survived into preservation.