Daryl,
Thank you for clarifying who took the picture and what kind of gun it is. The reason I posted that picture is because on another board, someone believed the the Ugartechea in question may have a similar self-opening mechanism to your Christophe.
I might guess that the Christophe is made by Jules Bury with parts from Britte. The receiver is marked with a crown over JB.
I suspect that also the case --a very nice gun indeed! Would you mind posting some more pictures of the gun and tell us a little about it? Also, is it possible that your photograph made it into a recently released book written in French by Laurent Bedu and published in Paris in 2009 called "Platines, les plus beaux fusils du monde"? The original poster of your photo claims he scanned it from that book.
I am going to take another step and write Ugartechea about this most unusual gun.