S= Super, as in top of the line model
L=Long, as in long body.
E=Esprainger, fuel injection
D- Diesel
Im not sure about the designation for K, although it might be Kompressor
a 300D would be a Mercedes with a 3 liter Diesel Engine
A 300SD is an S class with 3 liter diesel
a 300SDL as an S-Class, 3 liter diesel, long body.
just some examples of the way I understand the letter designations
