For your car:
300 means a 3 liter engine, S means a luxury model, and D is diesel engine
The W116 is the basic body style, which I think was shared with the 380SE in your vintage. This meant 3.8 liter engine, S for luxury, and E for "einspritzen" or fuel-injected.
Do not take this too far, as the designations lose their logic between years and keeping it straight is an art unto itself. Sometimes, S was for Sport and L was for Light or Luxury. Then there was the 250C which had a 2.8 liter engine and a C for Coupe.
If you get the Adsit catalog, you will find a good description of body and engine designations.