Looks like there should be a plunger/pin with a section of female thread to engage the screw - bronze maybe? It should be fairly easy to fake one, get the required form by pressing e.g. some epoxy putty onto the *well oiled* thread through the *well oiled* hole on a well-fitting dowel, allow to harden some and remove while you can, then when it's fully hard use as a reference to make a copy with a Dremelloid or similar, or if you want to be flash (and your lathe will do threads that coarse) machine a matching end on the plunger (might be an interesting workholding challenge, clamp on a V-block on the faceplate?) with a matching tip in a boring bar (expect horrible amounts of chatter and a banging interrupted cut!).
It's probably (and looks like) a square thread though, not an Acme, an Acme could push the plunger out of engagement under load thanks to the flank angle, unless there was a positive stop holding it engaged?
Dave H. (the other one)
Oh, one more thing - if you do it this way, rotate the screw so your plunger has the most "teeth" on the end (so most thread "grooves" will be visible through the hole) when taking the pattern mould/impression, to encourage a bit of strength!
