Since Mercedes-Benz design is a continuous process of evolution and development, the newest developments are put into production as soon as they are available, rather than waiting for a new model year. While arbitrary cut-off chassis numbers are chosen each year to designate the onset of a "new" model year, it does not necessarily mean that a 1974 car, for example, is radically different from a 1976 car. Especially it does not mean that an "old" 1974 car is obsolete.
All this is great for the owner, but it presents a problem when ordering parts. The solution is found in the comprehensive Daimler-Benz identification plates, found in various places, but usually under the hood and on the door posts. Consulting the illustration pertaining to your model, find the location of the chassis number plate, type plate and engine number. With these numbers, plus other sub-system numbers, you'll have all the information you need to identify your car.
When ordering parts, give the complete chassis or engine number, plus the number of the concerned area. The engine and chassis numbers are quite complicated, but each is a 6-digit number divided by a decimal point. The 3 digits to the left of the decimal point identify the basic chassis or engine, and the 3 digits to the right identify specific modifications. This is the reason you have to give the entire identifying number when ordering parts. The Model/Engine identification chart gives a picture of the model/engine/chassis combinations that have been imported to the U.S. that are covered by this guide. Some models, of course began production before 1974 (the first year this guide covers). For space reasons, these models are covered in another guide, Mercedes-Benz 1968-73 Repair & Tune-Up Guide.
Be careful with the model designations. Just because the same number is used to identify a car, it does not mean components are necessarily the same. For example, the 300CD and 300SD do not share the same chassis design, nor do the 280CE and 280SE.