In 1978 the first 55 300SDs were built. Their transmissions and those
that followed were the basic 300D 4 speed but modified to start off in
1st gear to give the car better off the line pick up.
This was done by holding the car in 2nd gear until the brakes were
released - to prevent creep at idle - and down shifting to first at the
lightest movement of the accelerator. An electronic logic board to
regulate this as well as transmission overload protection was installed
in the 116s' right foot well - that's part of the system.
I believe the turbo's transmission was also strengthened internally to
handle the engine's higher power.
In the mid 80s the torque converter was modified from the initial design.