According to the Price Card for 1970 models, Mercedes did not use "Convertible" when referring to the SL.
The card shows an SL Roadster, a Coupe, and a Coupe/Roadster but no SL "Convertible".
The only Convertible on the card is a 280SE.
The price difference at that time between a Roadster and Coupe /Roadster was $422, about 6%.
Richard M, NYC