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 

Note:  Screws sold separately and can be found here.