The class designation is always hard to get the grips on...

As far as I know, the name "E-class", "S-class" and so on was not used before 1994. From early eighties to this year, all the small-sized cars were named 190, the rest had names according to engine size. If it had an S, it was the large sedans (or coupe), if not i was the mid-sized 124. The letter "E" told that the engine was fuel-injected, "D" was for diesel, and "C" coupe. "T" first ment stationwagon, "T" last ment turbo (this was only available on the diesels). The number 300 means a 3-litre engine, 250 a 2,5-litre and so on. The exeption is the 190, which never came with a 1,9-litre, but on this model the engine size is written after the model designation, for example 190E 2,3 (190-model with a 2,3-litre fuel-injected engine)

It's not right to say that they 'moved' the letter E from behind the number to before. In 1994 Mercedes dropped the E (meaning fuel-injection) and introduced the three main 'classes', C, E and S. For example, a W124 with the 3,2 litre fuel-injected engine was named 320E in 1993 and E320 in 1994 (but the E means a different thing!). A 3-litre diesel went from 300D to E300 Diesel. To call a 1990 300E for E-class is wrong, but still a lot of people do.

What I have written here is based on the European models, but I'm quite sure the same applies to the US marked.