Friends and many members always ask about the alphabet game in the Mercedes-Benz nomenclature; here is a simple guide:
W (W123, W124, W126, W140, W210, W220)=
Wagen (wagon) to distinguish tourism cars from trucks.
E = Einspritzung (fuel injection)
To distinguish regular aspirated (carburated) engines from fuel injected ones.
D = Diesel
L = Lang (long wheelbase chassis models, like 500SEL)
Also, a "V" was used as an inside calling for these models, for example a short wheelbase 140 S-Class would be referred as S500W and the long wheelbase as S500V.
Also, the L was used as "Leicht" (light), like in the case of the 300SL, where ir refers to it a a "sport light" version model.
C = Coupe (two door models)
T = Touristik & Transport (sometimes mistakingly called "touring", but the term was well accepted in the inside trade). In plain words: station wagon (5-doors)
In a few instances, the "T" was informally used to identify "Turbodiesel" models, in the case of the S123 300TDT (as it was known inside factory, the public knew it as the 300TD Turbodiesel.
S = Here it gets confusing, as the "S" was used for several things. Mainly it refers as "Super" (top of line models, like in the S-Class -W126/W140/W220-).
In other cases it meant "Sport", like in the aformentioned case of the 300SL.
In the case of some models were an "SS" term was used, the first S stood for Super, and the second S stood for Sport.
In chassis designations, the "S" is used to identify "station wagons" from regular 4-door sedans, for example, a four door 1988 300E would be a W124, while the wagon would be called S124.
K = Kurz (short chassis) in earlier models. Later models is for "Kompressor" (supercharged engine). By the way, the "S" has never been used to signify Supercharged.
A = used in chassis designations for convertibles, like the A124 (a 124 series convertible, i.e. 1993 300CE-24) In some cases it was used to identify "automatic", but never officially, just as a reference.
R = used to identify "roadsters", as in the case of the R-129, body style of SL cars (300SL/500SL)
Small letters such as b, c, etc. were used to identify further models of a previous series, like the 300b.
It is important to note that the letters, throughout the years, have not always been logically used. There are some exceptions.
Hope this was useful for someone.