The outside of mine is left hand thread and it tightens the center fixture for the lead screw..........then the adjustment screw( right hand) goes into it
the outside double tapped nut is left
hand to remove it from the bottom part of the vise section, the center
part of the nut is a right hand tap for the slotted screw that adjusts the bearing . My vise in that area, the double tapped unit I think has a burr problem and I am still investigating