Can you post some detailed pictures that show what the lock mechanism looks like? Is the lathe the really old medium gray or the newer blue color or the even newer dingy-not-quite-Tide-white?
Also, is it creeping when the quill is cranked to apply pressure? Or just plain loose all the time? The best solution is a T-slot nut custom fitted to the gap between the ways, but it probably has a round disk that has worn over time or worn the bottom of the ways. Another possibility is that the cam or the sleeve or the holes could be worn since I don't believe that they were bushed.