I think that this topic has already be beaten into the ground but I'll chime in anyway.
On cars with US lights, separate fog lights came sometimes on cars that did not have the fog light built into the headlight door. Earlier cars have the fog light built into the headlight door. So extra fog lights combined with headlight door foglights seems odd at best. This confines the disussion to 280 SLs I think. So for US 280 SLs I think that the correct configuration is round Hella chrome fog lights on top of the bumper, which in my opinion don't look good, but that's a separate issue.
On cars with European lights - I don't know if fog lights were built into the headlight door so I won't comment.
My observation is that in photographs, cars with European lights, if they have fog lights the lenses are clear and smooth on the inside and outside, while on cars with US lights, if they have fog lights, they are a little frosty and ribbed on the inside of the lens.
I believe that the correct US optional round fog light is the same as that found on early 114/115 sedans.
The original post showing the under bumper rectangular fog light is I believe incorrect for all applications. That's the fog light from a 107 and other sedans of the 70s and 80s. (post 113, post 114/115)