Here's what Wikipedia says on the subject, although the expertise of the author is unknown.

The R107 took the chassis components of the mid size Mercedes-Benz W114 model and mated them to the larger engines from the S-Class. The W 107 chassis is also referred to as "R 107" for "Reihe" (series). The series comprised SL and SLC models.
The SL variant was a 2-seat convertible/roadster with standard soft top and hardtop. The SLC (technically C107) derivative was a 2 door hardtop coupe, with usable rear seats and in effect an SL stretched 10 inches (254 mm) with a fixed roof.