I don't have experience with Van Norman so keep that in mind.
However, looking at the photos and your description, it sounds like there should be some mechanism to trap the screw that's not operational. The second photo shows what looks like an oil cup at the top of the casting housing the shaft and a smooth shaft section exposed.
I suspect if you push the ram casting back physically to take up that slack and look underneath you will see some shaft collar missing its pin or screw. It might have been pushed up the shaft for a bit so keep looking and wind back a little more if needed.