Interesting to see a red plate on a green liveried engine. Red plates were usually associated with engines painted in lined black and, when the decision was made to repaint some mixed traffic classes into green, it was usual to repaint the number plate background black at the same time as the engine was repainted. Note that the nameplate has been correctly repainted My thinking is the loco was lined black with red backed plates, went for works attention and they took the cabside off, put it to one side, repainted the engine and nameplate, then re-fitted the cabside once the repaint was done. They certainly left the nameplates in place on the splashers at Stafford Road when they lifted a boiler so that would be repainted when the boiler was re-attached. There is a picture of it repainted at Swindon Works on October 21st 1962 but I cannot make out the cabside colour.

Trevor Tremethick says "
I think this is a 'local variation'!

6921 was a Laira engine in the 1960s (in green livery) and I think her numberplates had a black background at that time. I suspect (as the nameplate background remains black) that the shed foreman accosted the artist and suggested that he re-direct his efforts to a more practical use! It would be interesting to see if the numberplate an the other side was also 'done'.

Note how much more difficult it is to read the number from any distance with a red background. That's why they were painted black! "