Allows a dial gauge to be fitted to most RepRap X carriages to check how level the bed is.
There are several of these on Thingiverse but none mount it in place of the extruder so that it has the same range of movement.
Use M3 nuts and bolts to tighten it around the stem of the gauge and bolt it in place of the extruder. It has M4 mounting holes 50mm apart and the clamp for the stem is 9.5mm. The design is fully parametric so these can be adjusted.