If I am seeing what I think I am seeing, the threads in your half nuts look pretty badly worn. In cross-section, the Acme thread is symetrical. Meaning that the peak and trough are the same width. It looks like the ID of the threads in your half-nuts is about half the width of the OD. If that's true, then the half-nuts are worn out.
On the labels, I've never seen a 5400 Series lathe and all that the manual shows is OUT and IN. So I don't know what else was there.