There was a period where it seemed MB tried to name cars with higher numbers for the nicer cars, but the numbers kept going up for the most part, so the plebeian cars kept getting bigger motors, with the /8 w114 / w115 eventually getting 2.8 liter (and even bigger diesel) engines, making them easily confused with the w111 and w108 and w116 280s, 280se, and 280sel (and hopefully not the w113 280sl too). By the time the w114 was called a 280 / 280c / 280e / 280ce the w108 and w111 cars were out of production. The w116 was available with the 2.8 m110 engine though, and was still called the 280s or 280se (and maybe 280sel, but I don't know).
In any case, you should be able to pick up on the fact that the "S" usually designated the body style for the top of the line cars. There are further distinctions and trim levels in those body styles, but I don't think your will find any "S" in the name of w114/115 cars.
I've tried to make sense of MB naming concessions, but honestly think it's pretty hard. Some apparent conventions of the period that will hopefully make sense...
S - usually the top of the line
L - Long / Lang version with the exception of the "SL", which is a convertible, never "Lang". I also don't think that you will ever find a long version that wasn't injected, so there's no need to worry about an S-class being a not injected long version, "SL". Uh oh!
E - Injected (not carbureted), but the "SL" was always injected, so no "E" necessary there.
C - Coupe
D - Usually a diesel
Don't even get into the SEb, and c, and d and stuff like that. I can't make sense of it (easily).
In any case, you are looking for /8 w114 / w115 bumpers. They were small from 68-73 in the US and for all euro / ROTW / Standard models. The 74-76 US cars got the hideous big bumpers.
This is probably irrelevant to your search, but the /8 coupes are all w114 (not w115). There are no diesel /8 coupes (from the factory) and all /8 coupes were 6-cylinder, which makes them w114, not w115. The /8 sedans are w114 if they are 6-cylinder, and w115 if not. Wikipedia covers most of this stuff pretty well. There you can see the model names you might want to search for on CL / ebay, etc.