This is a drill hole jig it is for installing pillow blocks. It makes sure that the holes for drilling are in the right place relative to your axis pulley rod.
The big hole is placed over your exposed bearing to confirm that the drill holes will be perfect
While you are there doing the pillow blocks, you might as well do the other jobs too. Because it is a pain to have to re-open one side of the printer and have to recalibrate the X-axis carriage again, it is better to do everything you might want to do all at once.
I think I used these pillow blocks
Put some Y-axis mounts in
You might also want to upgrade the stepper mount while your at it.
To recalibrate the X axis carriage and make sure its square when you put it all back together use-
Let me know if I have missed something. The Jig was made with FreeCAD and the file is included so it should be easy to modify this if your needs are different.