[sirbob]

Thank you, everyone, for your helpful replies. I am totally satisfied that this has been resolved as far as I was asking.

I must say, however, that it has evolved into a very interesting thread concerning interpreting the effect of filters in black and white photography of the time.

I must especially thank Klenchen for sugesting that the aircraft was a G-2, as this was also my opinion but I did not wish to contradict the learned author of the book.

As for Graham Boak's sugestion that the upper wing is 79, I have now tested that idea by super-imposing a sample of the wing over the natural sand, and there is very little difference in contrast so I presume it to be correct. Thank you, Graham, for your assesment of this optical illusion.