If it is like the last Logan that I managed to find a section view of the spindle on (and I don't recall the model), the set screw in the smallest pulley is a set screw in fact, and secures either the small spindle gear to the pulley or secures an intermediate part to which the gear attaches. The set screw in the (usually middle) larger pulley is an oil plug. There was nothing shown in the largest pulley on the drawing that I was looking at. In any case, there must never be any set screw in the spindle pulley on any back geared lathe that locks the pulley to the spindle. When in back gear, the pulley turns much faster than the spindle. If you manage to force the oil plug on down and contact the spindle, the first time that you use back gear afterwards, you will either stall the motor, break teeth out of the back gears, or ruin the spindle.This is true on at least all Atlas, Logan and Clausing machines except for those noted below. It may well be true of larger machines but I've not studied them. It is a known but unfortunately not well known problem with all of the Atlas machines except for the 9" and the 10A, 10B and 10E, none of which came with back gears.