I'd stick with my original timeline of 96 being the first year, but only certain models. The SL320 and S320 they developed the 722.5 5 spd overdrive just for those models, until as I stated, 96 is when they went from the .5 to .6 on these models, including the V8 and V12. I can hopefully get a good timeline for when the different models went to the 722.6.
You can definitely tell if your car has the 722.6 if it was the rocker switch on the shifter plate, one side marked "W", one side marked "S", W is for winter, which gives a second gear start and a different reverse gear ratio, and "S" is "Standard", which gives a first gear start. Also a 722.6 has a very distinctive type of dipstick tube cap, which has a one-time use locking tab, and there is also no dipstick on the .6, just the cap.