These chucks don't use a locking nut to vary their position on the headstock thread. As the previous reply has intimated, they have to be fitted all the way down the thread. so the back of the chuck is tight against the face of the headtock shoulder. What you are referring to as a locknut, is actuall the thread adaptor which also must be fitted all the way into the chuck body. The fact that you managed to mount the chuck only part way onto the spindle thread and got no wobble was pure luck and I am amazed that you were that lucky. That said, if you are running the lathe with the chuck only part way onto the spindle, you are running a real risk of things bending or even coming off the lathe.
Do it all up properly and your wobble woes will disappear (unless of course, the previous owner was doing the same and managed to bend the spindle! I hope not.) Hope that helps.