The original single lever process simply energized the cranking engine's starter motor when you pushed it forward. It took a pretty stout push but that's how it worked. The trouble with it was if you pushed too hard when disengaging from the Diesel it was possible to re-engage the cranking engine's starter with the then running (at about 6000 RPM) cranking engine.
Normally this forward motion on that single lever was taking place when the diesel had just started. On the 70D the cranking engine was governed at 6000 (I think they calmed them down a bit on the 720 and 730) and he had just finished his task of starting the diesel so it was going full bore which was NOT a good time to ever bump the starter drive pinion into the flywheel.
They re-designed them starting with 7026869. Earlier 70's could be converted using the AF 2305 starter pedal assembly kit. I have one such "converted" tractor.
Hope that helps.