Construction of the first batch of this class was authorised by Midland Railway Order O/3942 dated 21 June 1911: 'Please put your work in hand in connection with rebuilding engines 483–522 with new frames, new cylinders and G7 boilers fitted with Schmidt's superheaters.'

Apart from the savings made by using the parts that were salvaged from the old engines, there was an added benefit in referring to them as rebuilt since the royalties due to the superheater company were lower for modified locomotives than for new ones.

Some of them duly passed into British Railways (BR) ownership in 1948.

Note: two of the above number series contain gaps so the totals do not tally.

On rebuilding, the Stephenson valve gear was retained but the following changes were made: