Second-gear starts have been part of several MB transmissions going back to at least the early 80's. For the 4-speed in the 560SL, and I think the earlier 380SL, there's an electric component to the control system that allowed Bergwerks to do their FGS module--a simple, effective electronic solution for a mostly-hydraulic box. For the successor, the only way to rig FGS had to do with the valving in the hydraulics. For the electronic in your 300SL, I've not seen any posting about a system to change to full-time FGS start.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)