I believe that all the gears in all the atlas lathes are 14 1/2 Pitch Angle. Here is something that I read on another forum.
The box is a transmission in two halves, the shaft the selectors ride on is what carries the power from the left half (16 pitch gears so they can mesh with the change gears), to the right half (20 pitch gears) which contain the stack gears and out put gear on the lead screw collar.