These are cable clips for the Modular Mounting System Arms and thing:3689842 by jtronics that allow you to neatly route any cables along the arms.
The clips are available for cable sizes ranging between 2mm and 9mm in 0.5mm increments.
You can also customize this part in Onshape and create your own custom-sized clips. Edit the configuration parameters in the upper left to your specific requirements. Then right-click on the part in the lower left and select "export".
Insert the hook between the 2 beams of the arm and rotate it 90 degrees. It's intentionally a very tight fit so that it stays in place. Use a pair of needle nose pliers to hold the clip and twist it in place if you have them available. It can be done by hand too though.
Clip the cable into the hook. The opening measures 80% of the cable diameter which is small enough so that the cable doesn't come loose and large enough so that it's easy to insert the cable.
Print the parts in the original orientation. The STLs are pre-rotated for printing.
The author marked this model as their own original creation. Imported from Thingiverse.