These are custom made OpenBuilds Tee Nuts.  They were designed to slide into slots with less hassle than the standard rectangular tee nuts.  They have angled corners to allow for a smooth transition.
OpenBuilds Tee Nuts fit V-Slot and most aluminum tee slot extrusion that has at least a 10mm wide tee nut slot.
Packs of 25
Low Carbon Steel
M5 Thread Size