This is a jig to measure the concentricity of a pulley / idler you could also use it to measure if the axle of a stepper motor is bent. It is designed on this dial indicator:
and is supposed to be mounted on a piece of 20x20 extrusion.
You also need a looped gt2 belt. I got one with 200mm length but you could also go a little bit smaller or larger. If you take a 200mm belt your piece of 20x20 extrusion should be at least 215mm long. Additionally you will need at least the following fixings:
6x M5x10mm head cap screw
6x M5 Hammer Nut
3x M3 Hex Nut
3x M3x20mm head cap screw
To use the Nema17 mount you will need 2x M3x10mm head cap screw.
If you want to use a second dial indicator mount your 20x20 extrusion has to have a length of at least 315mm and you need 2 additional M5x10mm head cap screws and 2 M5 hammer nuts.
I usually mount two pulleys side by side, mount the dial indicator and then turn the belt by hand. This gives much more controle about the measuring process and you have more time to read the exact values from the dial indicator. Then I read the deviation in both directions and add them up. This way I got a repeatability of about 0.02mm. Note that you won't get a deviation of 0 because the pattern of the teeth can be measured on the back side of the belt as well. But as this pattern is very homogeneously it is included in every measurement the same way and does not really matter.
The files "Dial_Mount_left.stl" and "Dial_Mount_right.stl" are only for convenience as you have all screw heads on one side of the extrusion and don't need to reach the other side to fasten / unfasten the dial indicator. If you use one dial indicator mount on every side you can measure two pulleys and only need to fasten / unfasten two screws to mount the dial indicator to the other side in order to measure the other pulley.
NOTE: This jig was design on the specific dial indicator mentioned above. This one has a mounting shaft diameter of 8 mm and the center of the measuring shaft is 8 mm above the ground if you place it on something flat. This makes sure, that the tip of the measuring point is at the same height as the center of the pulley. If there is much need for it I could revisit my fusion360 model and include a few parameters so everyone can adjust according to their own dial indicator dimensions.
UPDATE: Added a slimmer version of the dial mount without the support underneath the main body. You can also mount this one on either side of the pulleys and still have all the screw heads facing you because it is symmetric. But bare in mind that this one is more flexible without the support for the mass of the dial indicator. The flexing might introduce a bit more deviation. This does however not matter if you mount it vertically.
UPDATE: Added a fusion 360 archive file with a few parameters to adjust to your needs.