This is a 10-32 threadable nut with a flat blade screwdriver slot at the top for tightening.  I designed it for a location where there is very little room to get a wrench around a traditional nut, so it was easier to make a part to get it up and out of the way.  You could probably also do 10-24 threads with no issue as the size of the hole is pretty close on the bottom.  

The hole on the bottom is 28mm deep, so you can fit a good size stud in this if you needed to.  Recommend when threading use a bit of cutting oil or just WD-40.  I printed these in PETG and they tended to gum up the tap a little bit without it.

I have these on an MSD Start and Step that is in a pretty tough to reach area under the dash

Printed in PETG with 100% infill for strength.