The numbers represent the engine and the letter represents the class of the car - the chassis really isn't represented in the letter anywhere - its just the SE/SEL/SEC/SD are W126's, and then 1986-1995 E/CE/TE/D/TD are W124, and the 1985-1976 E/CE/TE/D/TD are all W123 cars.

For the W126, the 500SEL 4.2 is called the 420SEL, the same way that a "500SEL 5.6" is called the 560SEL. The 500SEL has a 5.0L motor. The 380SE/L, a 3.8L. The 300SE/L, a 3.0L.

For the W124, the only discrepancies with the numbering were the 300E with the 2.6 or 2.8, and I think some might have come with a 3.2, this was in the later years of production - I believe it was 1992-1995 that the 3.2L motor was in production but they still badged the cars 300 - I think they started with the E320 badge in like 1994 or so. Same with the E400/420E - from 1992 to 1995 the car had the 4.2L, and didnt get the E420 badge until the late years (94-95 I think). The E500 has always had the 5.0L M119, and for the W124 diesels there was the 300D in 1987 that has the 3.0L turbo six, and then from 1990-1993 it was called the 300D 2.5 with a 2.5L turbo five, and then in 1995 it was the E300D with a non-turbo six.

If you do a google search for W126 or W124 or W123, and read the wikipedia articles on them - there are charts that show which cars got which motors.