[sirbob]

Thanks for the replies so far.

Surely the light/dark demarcation line is too sharp and straight for it to be oil/exhaust stains. Also, as Don points out, the number "1" is definitely painted OVER the dark colour. The cross and white band also. (Not too sure about the chevron, it looks touched up to me). And look at the aircraft at the top left behind the sand bags - it has the same paint scheme with the same sharp demarcation line.

IF it is a 78/79 scheme with a filter making the blue 78 look dark, what about the dark colour of the upper wings - surely this would be expected to appear the same as the UPPER fuselage, not lower?