The 64 Electrical Assembly Manual shows the clip with part number 370805-S100 (it's a lot easier if you list the part number of the item that you find in the assembly manuals) as being attached with a screw, 42127-S8, to a metal body piece tucked in the upper left corner. Your nut and screw/bolt in the picture may be misleading. There is another clip shown (different part number, 377904-S) attached with a nut (34655-S7) to a stud to the right of the vent.
Note: Screw 42127-S8, is shown with a quantity of 2, but only one appears on the drawing.
Jim