Yes, I am very familiar with the effects of wear and cleanup efforts on paint, too. I was a crew chief on B-52Gs, starting before the fall of the Soviet Union. This picture did not seem to fall within any of the "reasonable" explanations offered up so far. Spilled oil? Seen it. Spilled fuel? Seen it. Wear and finish changes from wear? Seen that, too. None seem to quite match that damnable difference between the DG and the OG visible in this photo...