Scribble on a Whirlwind photo

Poking around the IWM collection, on a photo of a No. 137 Squadron Whirlwind being bombed-up there’s a scribble just under the port engine:
http://www.iwm.org.uk/collections/item/object/205094154

I was wondering if anyone might happen to know why? A censor trying to conceal the location of the photo (I think 137 was at Manston at the time)? It doesn’t seem to properly cover anything, most curious…