After looking at some pictures on the web, it looks to me that the 05 S55 has a badge behind the front wheels like the V12's do, is this correct? Or possibly just a S600 photo by error?

 

The turbo and supercharger equipped AMG models have those badges.
C / SLK 32 -- V6 KOMPRESSOR
E/S/CL/SL/G 55 -- V8 KOMPRESSOR
S/CL/SL 65 -- V12 BI-TURBO


S/SL/CL 600 only has V12 wriiten on it, but its also a bi-tubo engine.