These are Du-Bro's "Permanent Mount" 4-40 Blind Nuts.
Zinc plated, metal construction.
Ideal for mounting engines, landing gear, cowls, hatches, etc.
The perfect mounting nut for hard to reach areas.
(4) Blind Nuts
Drilling a 4mm hole to install
Diameter: 8mm OD of flange
Thread Size: 4-40