Hi Nicolas,
I don't know an awful lot, but I read a few things here and there.
From the catalogues I have, the Ref 2499, which is the two subdial Chrono without date of the more common Cambre Carree 2498 always appears only with the white dial. So far, every 2499 with black dial was a Sultan of Oman commission. I only have seen one or two in Japan on offer that seemed to have a standard caseback, but I cannot be sure. However, often Japan gets different dial versions, something I have seen on other GP models, so quite possible.
The Sultan of Oman special commission is something that seems to have happened a few times, not only with GP. I'm aware of at least one Patek and I think even some others. I understand the Sultan of Oman is ordering a batch of watches with his coat of arms to give them away to government employees or military officers. What a nice dictator.
So I'm waiting for my Three-Golden-Bridges Angela Merkel commission. Bugger, I don't work directly for the government... on the other hand if I consider all the taxes I'm paying then I do work for the government
CC