Isn't that the same 5-speed auto (722.5) that was offerred in the EARLY R129 6-cylinder models (e.g., 1990-1993 300SL)?
It is a hybrid hydro-unit with an electronically actuated 5th gear overdrive. Basically an afterthought to the standard 4-speed hydro unit (722.3), to bring the revs down on the roadster's high rear diff (3.69).
The roadster weighs so much, that by only going to a higher diff was the acceleration deemed decent by the public.
Although you would need a custom-length driveshaft (approx. $500), I don't believe the tranny's black-box would be too hard to retro-fit, as long as your got the entire wiring-harness and 1 or 2 sensors, esp. since I don't think it even talks to the CAN.
I've been contemplating this with the 360TE AMG wagon since with a 3.07 and 3.27 diff, the highway revs are TOO high.
1988 360TE AMG