And to take it one step further:
S - Luxury
E - Fuel injected
L - Long Wheel Base
Also, I believe that the W108/109's (109's should be 300 SEL 6.3 and 6.9's)were the last of the hand built Benz's, any higher chasis numbers utilized some degree of automation.
Before long you'll be recognizing and classifying Benz's by chasis numbers - Ooh, look at that clean 108 (refernce to Suzanne), etc.
73 280 SEL 4.5