I have seen this photo many times (it's for sale on eBay today) and it only just occurred to me the significance of it. Perhaps I'm way out in left field but hear me out.
'Thou Shalt Not Steal.'
Why was David holding that number? Why not any other of the numbered commandments? And take a good look at his expression.
What did David have stolen from him while "The Alexander The Greater Affair" was being filmed in 1965? His wife.
After filming "The Great Escape" in 1963, the McCallums moved to the U.S. for the filming of "The Greatest Story Ever Told" in 1965, Jill was more than happy with the move so she could be closer to her new love interest- Bronson. David even said so in a radio interview.
I suppose David could have held up number 7, 'Thou shalt not commit adultery,' but that may have been too overt on his part by doing so. It's also possible that the photographer handed him the number 8 and said, "Here, hold this" for the shot.
Anyone else have an opinion about this? Just wondering.