Over the long (for its time) production run, Traction Avant styling changed little. When researching this post, I found a number of images on the internet whose model year identifications were wrong -- in at least one case, it was off by more than ten years! Which is why the model year identifications below need to be taken with caution; I tried my best given the sources in my library, but I could easily be off by a few years. Citroën mavens should feel free to provide corrections in comments.
Citroën 7A of 1934 or 1935. For 1936 grill frame brightwork was eliminated and headlights received curved lenses.
Citroën Traction Avants had louvres on their hoods rather than doors. Headlight assemblies are painted rather than chromed.
Citroën Traction Avant styling the year the new DS line appeared. Horizontal front bumpers were on 11 CVs in 1951 and on 15 CVS the year before.