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taken from http://www.geocities.com/SiliconValley/Pines/6633/w123models.html

The Mercedes-Benz W123 Models
The models are divided into two main categories here; gasoline driven and diesel driven models. The models are marked by a number and sometimes one or more trailing letter. The number represents the size of the engine in liters (the exception is when a larger engine is installed in the same model at a later date), like the 200 has a 2.0 liter engine and 230 a 2.3 liter engine. The letters have the following meaning (some of them are not used for the W123 model);

S – Sports model
E – Einspritzung (fuel injection)
C – Coupe (two door)
T – Transport und Touristik (station wagon)
L – Lang (long)
SL – Sport Leicht (sport light)

The body has the suffix W and the gasoline engine the suffix M and the diesel engine OM. This represents the following;

W – Wagen (car)
M – Motor (engine)
OM – Öl Motor (oil/diesel engine)