You've got me at it now. According to Wikki:
"After initially using letter prefixes (E for ex-LNER, M for ex-LMS, S for ex-SR, and W for ex-GWR locomotives, as used for other inherited rolling stock), a numbering scheme was decided on in March 1948. Generally ex-GWR locomotives retained their numbers (and hence were able to retain their cast brass number plates) and it was decided to add 30000 to the Southern numbers, 40000 to the LMS numbers and 60000 to the LNER numbers. There were some exceptions though."
However, Donald's engine is an LMS Ivatt 2 design. Wikki describes these (see Below) as being initially built for British Rail's London Midland Scottish operations in the North East of England but this particular engine is part of a batch that was allocated to BR's Western Region - and therefore presumably, one of the exceptions referred to. It looks like it may have finally found its way home.
"The 38 members of the Darlington-built batch (46465 to 46502) were allocated to the Eastern and North Eastern Regions of British Railways. The final 25 (46503 to 46527) were built at Swindon Works and, being allocated to the Western Region, consequently carried the GWR-type vacuum ejector and firehole doors."