There is one unique feature of those Chrysler pushbutton automatic transmission systems, I believe. Up to that time (and for long afterwards) the connection between the selection lever (whether on the floor or on the sterring wheel shaft) and the control devices in the transmission were mechanical - levers and rods, SOMETIMES Bowden cables. The Chrysler systems were electric connections from the pushbuttons to servo-motor actuators in the transmission case, except perhaps for the Park lever. Those may have been the first automotive use of electrical connections for such functions.
That excellent review of details of government regulations above included the specification that Low speed should be limited to a max of 25 mph. That reminds me of a review of that Chrysler system I read on those days. The reviewer wanted to know what would happen if you accidentally pushed the wrong button while driving - an easier error to make than moving the shift lever. He reported that, if you hit Reverse at low speed, the car slowed down to a stop, then shifted to Reverse and started to back up. But at higher speeds the car simply ignored that Reverse button.