It's nothing special frankly. All double gears are made that way. The larger gear is engaged to the tranny drive gear all the toime and the smaller gear slides in/off of it. Don't be too sure about the 2:1 drive. It is only the output shaft ratio that is important. They are all over the board, from way under the engine speed to way over it. There is a number on the side of the larger driven gear and that will tell you what it fits if you have the chart. They can be configured in a good many ways for left side or right side, shaft above ctr or shaft below ctr. Muncie, Chelsea, Spicer, Tulsa are the common ones. Only Muncie and Chelsea (now Parker Hydraulics) survives.
