The other has me puzzled - 0.492"/12.5mm thread OD and a 2mm pitch. The nut is 21mm AF.
Finally got the beast plugged in and discovered that is all seems to work. The middle bottom switch I still don't fully understand - in one position the vertical spindle motor does not run at all!
This is more complicated than you think, there are other things to consider. Yes, looks like two nuts and an adjuster, made to compensate for nut wear. The problem is though, the lead screw will be worn as well, and if you close the nut up too much in the mid travel it will bind at the ends potentially. Also worth looking at the staked nut on the end of the lead screw - its possible you have some bearing wear you can take up here as well.There is a lot of backlash on the X axis, about half a turn. I see on the leadscrew this large brass nut with adjustment slots. Is this likely to be for backlash adjustment?
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I will try playing with the brass adjuster
The staked nut will be underone of the handwheels, as @Agroshield said on thethreaded end of the lead screw.The advice above is good. The thing to take away from it is there are _two_ sources of backlash: (1) between the screw and the nut (2) between the screw and the table. Sometimes the second of these is easier to fix than the first because its resolution is not affected by different wear along the length of the screw.
Normally at one end of the leadscrew there would be a thrust bearing arrangement holding it into its bracket. At the other end, the leadscew is axially free floating in the bracket. That is so it is not over-constrained. Check the thrust bearing has no slack in it and that if there is a powerfeed fitted that a spacer or shim has not been missed. After that part of the system is tight, then the brass adjuster is the next thing to examine.