The W140 series S-class was manufactured from late 1990 to late 1998 for Germany and the rest of the world, except the U.S.
In the U.S., the W140 was offered for production years 1991 through 1999.
Basically, the cars are very similar having all the same body.
In the middle of 1993 (1994 production year for the U.S.) the line had a major facelift (mostly cosmetic changes to make the car look less bulkier).
Badge designations for early production cars (1991-1993) were as follows:
Then, from 1994 on, the badges were simplified as:
(You had to look at VIN number or the car itself to see if it was a short (previously SE) or long (SEL) wheelbase model).
From 1991-1993 the 300 SE/SEL were really all fitted with a 3.2 liter engine, hence it should really have been badged 320 SE/SEL, but it wasn't.
Hence, a 1991 600 SEL should be almost identical to a 1994 S600. The 1994 S 600 would have the facelift and minor mechanical updates.
Hope all of these can help you to clarify the matter.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995