This is a precision X belt tensioner for the Replicator 2. It snaps on the rear X rod and rests flush to the left. It is held in place and it's movement tracked by a cable tie. Tension is granularly adjusted with the twist of a screw. It requires;
1 18mm m3 (roundhead is best)
1 16mm long 3mm OD rod
1 3mm x 6mm x 2mm bearing (VXB 623 will do nicely)
1 2.5mm wide cable tie
*Note this is not designed for the 2x and I do not know if it will fit one.