The old Atlas that I had, has a full length keyway on the leadscrew.
My current lathe does not have said keyway. My thread dail has 4 major divisions and uses a 32 t "gear".
Pierre
It's my understanding that to reduce costs, some lathes will combine the leadscrew with the power feed by doing this (having both threads and a keyway). Larger and/or more expensive lathes will have separate leadscrews and power feed shafts.