These Tee Nuts have a spring loaded ball that prevents unwanted movement of the nut in the slot. This allows for inserting nuts in the verticle position as they will not slide from gravity.
They are highly desirable to allow for adding tee nuts in a pre-built design. Just slide them in at an angle along any point of the rail.
These tee nuts work by being angled and inserted under the lip of the extrusion slot.
M3 tap.