There is a photograph in John Rawlings's 'Fighter Squadrons of the RAF" showing a line up of 45's Camels with "S", B2494 being the odd one out with its serial roughly painted on the fin.
My guess is simply that B2494 was fitted with a rudder ( possibly "borrowed") from another aircraft in the field for whatever reason , such as a ground handling incident. This rudder serial of course then having been being painted over.
As such an early example of the long ongoing history of "Christmas Trees"and "Cannibalization" in the Service.