Some time ago, I changed the external shape of the threaded and cam-locking STEMFIE nuts to achieve a straightforward way to differentiate them. After gathering more experience and feedback from the STEMFIE community, I changed the design of the threaded nut to make it smooth. This makes it clearer to see the difference between them when looking at a pile of both kinds of nuts. In other words, the threaded nut is smooth, and the cam-locking still has its longitudinal slot.
You can download the updated version of the nuts (.STL and .stepZ formats) from this link. The STEMFIE Project is designed in FreeCAD and Blender. You can download this FreeCAD scene file to see both nuts' differences.
This toy is unsuitable for children under three years due to choking hazards! 
