The first car`to use the "E" designation was the 1959 W128 body 220SE, I should know, I own one.

The 300d sedan, the 300SL, and the 300Sc were fuel injected, but did not use the "E" designation. The "d" and "c" designation refer to the evolution of a particular model. For example, the W111 body 220SE is know as a 220SEb to distinguish it from the first W128 body 220SE, and the 300 sedan series cars from the '50s had 300b, 300c, and 300d designations. The 300S coupes and cabrios were also designated as 300Sb and 300Sc cars. Mostly to denote running production changes.

The first 300S cars had Solex carbs, the last of the 300S cars had fuel injection and were known as 300Sc cars. The rear bumper had the word einspritzung cast into the center, the German word for fuel injection.