I used 2 speeds until the early 80's and all we had was the early style. The early and late that Eaton books refer to was due to the a change in the shifter box. The early used a motor that reversed by reversing polarity and they had points in the box to stop the motor when the shift was completed. An ajax screw controlled the shift lever. They were simple and worked well. But in daily use, water would get into the box and the points would corrode, the ajax screw would gall and the bottom bearing would fail. Sometime in the late 80's, Eaton introduced a new style, that used electronic circuitry which was supposed to be better. I have never owned the new style. The old and new style are wired differently, the motors and none of that stuff is interchangeable, but Eaton did sell a unit to upgrade from the old style to the new style. This didn't interest me because already in the late 60's I had decided air was a much better way to shift a 2 speed.
The 1700 series was the most popular in those days. It was rated at 18,000 and no one knew why. 18k was all most states allowed on a single axle in those days. The 20 series was used in higher horsepower applications.