Capital K stands for Kurz or Kompact, not for kompressor. Accordingly, SLK denotes Sport, Light, Kurz/Kompact. A supercharged car could be termed thus: SLK230k, the little k identifying the presence of a supercharger.
L has three possible meanings. Lang is most common: 560SEL. Light is next: 500SL. Yet more recently it has stood for Luxe: CL600 (Coupe, Luxe).
The Mercedes nomenclature seems to have become rather confused since 1993/4, as hinted at by David C-Klasse. A-, C-, E-, M- and V-Class cars derive their letter from nowhere in particular other than a hierarchy which gives smaller cars a lower letter (although it has been suggested that E-Class means Executive Class and C-Class means Compact Class).
Yet these models sit alongside G, SC, SLK, CLK, SL, S-Class, CL and SLR cars whose letters are meaningful descriptors of their character.
Finally, am I the only person who thinks SEC is far better than CL? I know fuel injection no longer requires identifying in the model designation and SC is taken, but still...
1999 SL 500
1993 320 TE
1986 500 SEC