Summary

You will need a 2.5M x 8mm screw, washer and nut, you probably have some left over from the original build.
The actuator bar sits astride the top edge of the Extruder carriage and utilises an unused screw hole. It activates the X-min microswitch when the nozzle reaches the edge of the bed and is adjustable. This means that you no longer have to enter a zero offset into the firmware for the X axis.
The object was designed in OpenSCAD so it can be edited easily if your extruder carriage is slightly different.
The print has no fine detail.