This was originally designed to be used with my webcam mount https://www.printables.com/model/1250262-neptune-4-webcam-mount-offset-for-logitech-c920 , as I didn't find any on Printables that were quite what I wanted. 

It is a ¼-20 thread;  9mm, 12mm, and 15mm long. Standard thread size for camera mounts.

Note: I've updated these files to be at nominal width on the bolt (previous files had been adjusted for a printer that was overextruding – sorry about that!)  
If you find these fit too tight, you can adjust in your slicer by scaling in X and Y (but NOT in Z!) by a few percent before slicing.