It’s Mercedes’ development code. Because each model (in this case, the E class) can have many iterations but are essentially the same (i.e. E300, E430, E43, etc) and a model can run through many generations that can be vastly different from the other generations, the development code is an easy way to categorize the different generations of the same model. The current generation of the E class sedans has a development code of W213 and the station wagon is referred to as S213. The previous generation E class is W212 and the generation before that is W211. Current S class is W222 and the preceding generation is W221. You get the idea. I believe every manufacturer has their own development code system.

C238 is for the Coupe and A238 is the Cabriolet...