OK, enough of the history lesson. Here's the crux of the issue. The finger points to the coarse 19-spline input shaft used through the '66 model year. The fine 24-spline input shaft (on right) arrived in 1967. These shafts engage inside the torque converter and because the 19-spline input shaft has a smaller diameter than the 24-spline shaft, it is possible to accidentally install a '67-and-up torque converter in a '62-'66 727. If you do, the car won't move an inch. More than a few Mopar builders have been stymied by this detail. Conversely, the input shaft diameter difference prevents '62-'66 converters from accidentally being installed in '67-and-up 727 transmissions. Thankfully, they just won't go in all the way. The parts in the middle are reaction shaft supports. While they both share a common 27-spline configuration, they are machined for different input shaft support bushings and are not interchangeable between cable and lever-operated 727 transmissions.