Airframes
Benevolens Magister
Now that I've seen it enlarged, I tend to agree. The tail-end of the mark exhibits classic static fogging of the film, but, as it (the mark) conforms to the fin leading edge, it may not be. It could be retouching, as the density seems to high for staining, but of course it could a smudge of oil or mud, accentuated by the tonal contrast of the print. There wouldn't be a problem omitting it, as I very much doubt it would have remained there long, if it is staining.