A retrofit assembly for a tool post adapted for rapid attachment and detachment relative to the compound rest of a lathe for adjusting the positioning thereof and for the presentation of a tool holder relative to the tool post consisting of a single operating body for tightening/loosening the tool post relative to the compound responsively to the turning of a stud on a screw means and for tightening/loosening a tool holder relative to the tool post responsively to the driving force of a manipulating device.