The 500SEC/600SEC were later rebadged as the S500/S600, I guess it was supposed to be obvious you were looking at a coupe, but it was confusing as the badging was identical to the sedans.
Then later, as it was pointed out, it changed to CL500/CL600, guess even Mercedes was confused by identical coupe and sedan markings. I believe the change was made at the same time the injection system went to ME1.0, which also coincided with the intro of the 722.6 transmission.
Albert, the 126 model badging never has an S prefix, it was always the engine size first, no "S" before the engine size.
Now the S Class coupe is given a unique model designation and model number. The S Class sedan is a 220 chassis, and what "amounts" to an S Class coupe is the CL Class, which is a 215 chassis. Prior to this, for example the 140 Coupe was still a 140 chassis designation, and also the 126 Chassis coupe was still a 126 chassis.