This is the problem we've had before, French "3"s and "5"s; this is Nieuport N.1539 (the serials are also on the wing tips) you can see the difference here between them. The Caudron looks like C.3531 to me.
I posted a couple of close-ups showing the numbers some time ago but can't find them.
Regards
Nick
http://www.airwar.ru/image/idop/fww1.../nieup17-5.jpg