Hi Mark, I had never heard of the 300SEL 6.3 being referred to as an 'S' Class. Sure, Mercedes used the 'S' letter on cars fitted with carburettors, such as the 250S, the 280S, or even the 220S. The lettering changed to SE or SEL (lwb model) when the engines were fitted with an injection pump. Take two of the 300 models - first, the FInnie 300 from 1962-1966. They were only ever designated 300SE - maybe even SEL, but I am not sure on that one. However, did they ever have a 300S ? You will find the answer is "NO".
There was indeed a 300S Adenauer - say about 1959.However, I believe that terminology only applied to a 300Coupe, or possibly 300Convertible. In those instances, the "S'" referred to "Super" or "Special" - those models were fitted with an injection pump. Conversely, I have never heard of a four door 300S Adenauer. Sorry Mark, it's a long winded explanation, but as I have previously said that the tendency nowadays is to refer to earlier "S - Class" models which is incorrect. Regards Styria