This is a dial indicator holder designed to fit over an MGN12 rail on top of a 2020 aluminum extrusion, as featured in the folgertech FT-6. My printer is a bowden style hotend and the indicator is below the nozzle, but you may need to remove the hotend on direct drive style extruders.
This tool assists in getting the bed trammed (level). Affix a standard dial indicator into the mount with an M5 bolt and nut and sweep over the bed with the tool on the rail, adjusting thumb screws for minimum indicator movement.
Please do this adjustment with the bed heated to printing temperature as the bed topology changes with temperature.