Now this is confusing. The puzzle picture can be found on a French forum without too much difficulty, but the person who posted it there describes it as a Citroën AC4, rather than a C6. Maybe there is better information somewhere else, but I haven't been able to find any.
Another member on the same forum reckons that the car is being used as a taxi, and the landaulet style body is by Chapelle et Jabouille, a coachbuilder from Levallois, in the Hauts-de-Seine area of Paris. I don't know if that's correct, but it's all I have been able to find.