The regulators with the split shaft were common to 28/29 cars that used standard (non scroll design) window cranks like Tudors, Pickups, Standard coupes. This is why those same handles, used all the way through production (and reproduction) have set screws as well as holes drilled for pins. With the 28/29 style regulators the point of the set screw would fit into the slot in the shaft and, when tightened, would spread the slot enough to "wedge" the shaft firmly into the handle. When the later regulators came into use, a pin would secure the handle to the shaft and both the shaft and handle would have holes for said pin. the difference in the regulator shaft design could have been due to cost or change in suppliers. I tend to go for cost saving as the early regulator had a thin, open semi circular gear that met with the gear in the handle mechanism whereas the later units had teeth cut out of a simple plate that mounted to the regulator arm. Precision gear vs plate with teeth cut out? Sounds like someone found a cheaper way to make the same item. Precision slot in each shaft plus the cost and tooling for a set screw vs 1/8" pin hole in shaft & handle??? No brainer.