Test cars are not so easy to find, some of them are the same, others are different and some of them may also be tested not on the road at all.. My source (The Mercedes Benz catalogue raisonne) probably reports these numbers according to the chassis numbers. I was trying to find all the different versions that were pictured: some of them are different in some small details (the logo, in the same version, is big in one exemplar and slightly smaller in another, the side window is divided or in one-piece, the tail lights are different etc).