I can't help much, but I did find this page that might be of some help. Don't know if your lathe looks like any of these, but the pics might be helpful. Good luck!
Engaging backgear on the Model 101 Atlas 6" lathe. At first sight there is no obvious way of releasing the large V pulley from its embrace with the spindle. The secret is the "external locking ring" on the end of the pulley; this is simply pulled outwards to uncouple it from the shaft and the backgear lever then operated as usual to mesh the respective gears.